Question

3. ____ the researchers developed the first flu vaccination recently was there a reliable protection against flu.

a) Since


b) No matter


c) Not until


d) Whether

Answer 1

c) Not until the researchers developed the first flu vaccination recently was there a reliable protection against flu.

• In the field of English grammar, 'not until' is used when the occurrence of something does not happen till a certain other action has not been occurred.
• In the question above, as there was not a reliable protection against flu until the first flu vaccine was made, hence the correct word to fill in the blank will be 'not until'.

#SPJ2

Answer 2

3. Not until the researchers developed the first flu vaccination recently was there a reliable protection against flu.

Hope it's helpful ☺️