A) They used to be the smartest in the class
B) My friends were able to cross the river
C) We used to went to L.A very often
D) They were able to clean the room yestarday
La forma correcta sería: We used to go to L.A. very often. Al utilizar la frase “used to” (solía/solíamos) el verbo que la acompaña debe ir en infinitivo.