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

Step 1: Analyze the first blank

The structure "it's best ___" requires an infinitive form. We use "to + verb" after expressions like "it's best," "it's important," or "it's necessary."

Correct pattern: it's best + to keep

Step 2: Analyze the second blank

The phrase "have got a better chance of" requires a gerund (verb + -ing form) after the preposition "of."

Correct pattern: chance of + finding

Step 3: Verify meaning

The complete sentence should read: "If your plane has crashed high in the mountains, it's best to keep close to the plane. Rescuers have got a better chance of finding the plane than one person alone."

This makes logical sense because staying near the crashed plane increases the likelihood of being found by rescuers.

Related Grammar Concepts:

Verb Patterns: Different verbs and expressions require specific patterns - some take infinitives (to + verb), some take gerunds (verb + -ing), and some can take both with different meanings.

Prepositions: Prepositions like "of" are always followed by nouns or gerunds, never infinitives.

Common Expressions: "It's best to..." is a fixed expression that always takes the infinitive form.

Final Answer: The correct option is "to keep / finding"