Foundation
English
Verb
Tenses
Question
Fill in the blank with suitable modal that denotes recommendation.
Be careful lest you ___ fail.
can
should
shall
would
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Solution
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Option A is the correct answer because 'should' is used to make offers and suggestions, and to ask for advice,or in rather formal contexts to make predictions and to talk about intentions or decisions, as in this case, the probability of failing is being expressed if care is not taken.

Option B is incorrect because it is the past tense of will. 
Option D is incorrect because 'can' is used to express ability or capability to do something, and failing is not considered as capability.
Option C is incorrect because We use 'should', not 'shall'  for advice and suggestions: