I worked on the problem ____ a long time and I worked it out____ myself____ last..

题目

I worked on the problem ____ a long time and I worked it out____ myself____ last..
A.for; by; at B.in; with; on C.on; by; in D.for; for; at the

答案

A
for a long time 很久
by oneself 某人自己
at last 最后