The major points of the lesson were quickly learnt by the class,but they were also quickly forgotten by them.

A. The class quickly learnt but forgot the major points of the lesson.
B. The class quickly learnt the major points of the lesson but they also forgot them.
C. The class learnt and forgot the major points of the lesson quickly.
D. The class quickly learnt the major points of the lesson but also forgot them quickly.

Given sentence is in simple past tense and it is in passive voice.

Rule :
Subject + V1 + Other agents