Question 21: Mark the letter A, B, C, or D on your answer sheet to indicate the undelined part that needs correction in each of the following questions: What we know about certain diseases are still not sufficient to prevent them from spreading easily among the population.
Change are to is.
What we know about some diseases is not enough to prevent them from spreading easily among people.
Noun clause:
What/ That + S + V + O + Singular Main Verb
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