This question tests your understanding of modal verbs and their usage in English grammar. Modal verbs express necessity, possibility, permission, or ability. The context is a magic trick, so we need to choose verbs that logically fit the situation where an impossible act is being performed.

Step 1: Analyze the first blank.
Speaker A says, "He is sawing that woman in half!" This is an observation of an action that seems real but, in the context of a magic show, is an illusion. Speaker B is contradicting or clarifying this observation. The action is impossible in reality. Therefore, the first blank requires a modal verb that expresses strong certainty that this action is not real or true. Let's evaluate the options for the first part:

• Ought not to be: This implies a duty or moral obligation not to do something (e.g., "You ought not to lie"). It doesn't fit here because we are stating a fact about reality, not an obligation.
• Can not be: This expresses impossibility. It is a strong statement that something is not possible. This fits perfectly because sawing a person in half and them surviving is impossible.
• Mustn't be: This is used for prohibition (e.g., "You mustn't smoke here"). It is not used to express logical deduction or impossibility.
• Must be: This expresses strong certainty or logical deduction that something is true. This is the opposite of what we need for the first blank.

So, for the first blank, can not be (or its contraction can't be) is the most suitable. It states the impossibility of the action being real.

Step 2: Analyze the second blank.
After stating that the sawing is impossible, Speaker B offers an explanation for what is actually happening. The second blank requires a modal verb that expresses a strong logical conclusion about the nature of the event. Let's evaluate the options for the second part:

• can be: This expresses possibility or ability. It is too weak. We can be more certain that a magic trick is, by definition, a trick.
• must be: This is used for strong logical deduction. After deducing the action is impossible, the logical conclusion is that it must be an illusion or a trick. This is the strongest and most fitting choice.
• might be: This expresses weak possibility or uncertainty (e.g., "It might rain later"). It is too uncertain for this logical deduction.
• might be: Same as above; too weak.

Therefore, for the second blank, must be is the most suitable. It expresses the strong certainty that the event is a trick.

Final Answer: The correct option is Can not be / must be.
The completed sentence is: "He can not be sawing her in half. It must be a trick."

Related Topic: Modal Verbs of Deduction

We use modal verbs to make deductions or guesses about a situation based on available information.

• Must: Used when we are almost 100% certain something is true based on evidence. (e.g., "She studied very hard. She must be confident about the exam.")
• Can't / Couldn't: Used when we are almost 100% certain something is not true. (e.g., "He hates flying. He can't be in London already; he was here an hour ago.")
• Might / May / Could: Used when we think something is possibly true, but we are not sure. (e.g., "I can't find my keys. They might be in the car.")

In the given question, we use can't for a negative deduction (impossibility) and must for a positive deduction (certainty).

Key Formulae/Theory

Structure: Subject + Modal Verb + Base Form of Verb
Examples:

• He must be a magician. (Certainty - positive)
• That can't be real. (Certainty - negative)
• It might be a trick. (Possibility)

The choice of modal verb depends on the level of certainty you wish to express.