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