This question tests your understanding of verb patterns and tense consistency in English grammar. Let's analyze the sentence structure:

The phrase "it's best" is followed by an infinitive verb form ("to + verb") to express what is advisable or optimal. Therefore, the correct first blank should be "to dig".

For the second blank, we need to consider the verb "stops" which can be followed by either a gerund (verb+ing) or a noun. The phrase "until it stops _______" requires the action of snowing to continue and then cease. The gerund "snowing" correctly indicates the ongoing action that will eventually stop.

Let's look at the complete correct sentence:

If you are lost in a snow storm it's best to dig a hole and sit in it until it stops snowing.

The incorrect options have these problems:

• "digging" - incorrect because "it's best" requires infinitive "to dig"
• "dig" - missing the "to" required for the infinitive form
• "snowed" - incorrect because we need the ongoing action "snowing" rather than the completed action "snowed"

Related Grammar Concepts

Verb Patterns: Certain expressions like "it's best," "it's important," "it's necessary" are typically followed by infinitives (to + verb).

Gerunds after Specific Verbs: Verbs like "stop," "enjoy," "avoid," and "finish" are typically followed by gerunds (verb+ing).

Tense Consistency: The sentence maintains present tense throughout ("are lost," "it's best," "stops snowing") for a general truth or advice.

Key Formulae/Rules

1. It + be + adjective + infinitive: It is [adjective] to [verb]

2. Stop + gerund: To cease an ongoing action

3. Present simple for general truths: Used for stating facts or giving advice