This question tests your understanding of modal verbs in English, which are used to express ability, permission, possibility, obligation, or advice. Let's analyze the conversation step-by-step.

Step 1: Analyze the first blank.
The first sentence is a question: "How ______ I get to the nearest bus stop?". This is someone asking for directions or instructions. The most natural and common modal verb used for asking about ability or possibility in this context is "can".

• "How can I get...?" is a polite and standard way to ask for instructions.
• "How may I get...?" is very formal and less common for simple directions.
• "How should I get...?" implies you are asking for advice on the best way, which is possible but not the most direct.
• "How could I get...?" is also used for possibility but is often more hypothetical.

Therefore, can is the best fit for the first blank.

Step 2: Analyze the second blank.
The second sentence is the response: "You _____ go straight and then turn to the left." This is someone giving instructions. The speaker is telling the listener what is necessary to reach the destination. The modal verb that expresses necessity or strong obligation is "must".

• "You must go straight..." clearly states the required action.
• "You have to go straight..." is also correct and means the same as "must". However, the option is "can/must", not "can/have to".
• "You may go straight..." gives permission, which doesn't make sense as a response to a request for instructions.
• "You might go straight..." suggests a weak possibility, which is not appropriate for clear directions.

Therefore, must is the best fit for the second blank.

Final Answer: The correct option to fill in the blanks is can/must.

The completed dialogue is:
"How can I get to the nearest bus stop?"
"You must go straight and then turn to the left."

Related Topic: Modal Verbs

Modal verbs are auxiliary verbs that modify the main verb to express mood or tense. The key modal verbs are:

• Can/Could: Used for ability (I can swim), possibility (It could rain), and permission (Can I leave?).
• May/Might: Used for permission (May I come in?) and possibility (It might be true).
• Must/Have to: Used for strong obligation or necessity (I must finish my work). "Have to" is often used for external rules, while "must" can imply personal obligation.
• Should: Used for advice or recommendation (You should see a doctor).

Key Formulae/Theory

There is no mathematical formula for this, but the logic for choosing the correct modal verb can be summarized as:

Asking for instructions → Use can
Giving necessary instructions → Use must or have to