помогите правильно выбрать форму глагола:

1.The clock_____,it's time to finish your studies
A-strikes B-has stricken C-is striking

2.They____ the problem now.
A-understand B-are understanding C-have understood

3.I___him today, but I saw him yesterday
A-didn't see  B-don't see C-haven't seen

4.He___this work before they came.
A-had done  B-has done C-did

5.When we___the telephone rang.
A-are talking  B-were talking  C-have a talk