I wrote to my parents once a week, ____ I was too busy with my

I wrote to my parents once a week, ____ I was too busy with my teaching work.
A、it
B、unless
C、when
D、as
【正确答案】：B
【题目解析】：考点点击本题考查条件状语从句。要点透析 unless意为“如果不,除非”,相当于 Fif not,其他三项均无此意。本句意为:我曾每周给父母写一封信,除非太过忙于教学。