Question

Fill in the blank with an appropriate modal:
Be careful lest you _____ fail.

• A. would
• B. must
• C. should
• D. could

Answer 1

Answer: (C)

should

• Option C is the correct answer because 'lest' has a negative meaning. Therefore, it should not be used withnot.The only auxiliary verb that can follow lest isshould.
  

For example -Work hardlest youshouldfail. OR Work hardlestyou fail. (NOT Work hard lest you should not fail.)

• Option A and D are incorrect because they are implied in the past tense whereas the statement is in the present tense.
  
• Option B is incorrect because 'must' is used to express necessity, but here the condition for failing is being careless, and hence not a necessary thing to happen.