Bunuel
Avgolemono or "egg lemon", is a soup or sauce made by mixing egg, lemon juice and broth and
heat the mixture until it will thicken.
(A) heat the mixture until it will thicken
(B) heat the mixture until it thickens
(C) heating the mixture until it will thicken
(D) heating the mixture until it thickens
(E) heating the mixture until it had thickened
OFFICIAL EXPLANATIONAvgolemono is a soup or sauce made by MIXING and HEATING. "Mixing" and "heating" are two steps in the same process and must be in the same format. Eliminate (A) and (B).
Choice (C) incorrectly changes to a future tense, and choice (E) incorrectly changes to the past perfect.
In general, when we want to describe a general process (an if/then situation that is true regardless of the time period, such as
To achieve X, do Y or
When it rains, my roof leaks), use the present tense.
D is the correct choice.