Question

Complete the sentence with the past perfect verb

though doctors (know) for decades that a change in diet could reduce the
symptoms of celiac disease, it wasn't until the 1950s that the disease was linked to wheat proteins.

Answer 1

the correct answer is “knew”

Step-by-step explanation:

this is because the word given is know but they are talking about something from the past and therefore need the past tense. The past tense of the work know is knew

Answer 2

Knew

past tense of know = knew