Question 23: ………..is caused by a virus was not known until 1911

That + S1 + V1 + V (main) +…
=> We understand the phrase: “That + S1 + V1” will play a role as the subject in the sentence, and V (main) will always be in the singular form. For example in this sentence: “That measles is caused by a virus” acts as the subject and “was not known until 1911” as the predicate (and you let to be in the singular form “was”
Translation: It was not known until 1911 that malaria was caused by a virus