He was sitting at the piano an old song. C A sing B sings C singing D sang​

С, т.к он пел в настоящем временем(окончание ing)

.1.-open the window.

-but i have opened the window.

2.i have seen the film.

3.he has opened the door.

4.they have cleaned the room.

1.they are in the school, aren't they?  

2.they aren't at home? are they?  

3.they do exercises, don't they?  

4.she can sing, can't she?  

5.he doesn't like soup, does he?  

6.there are a little bear, aren't there?  

7.it is a cake, ian't it?  

8.you will go to the cinema, won't you?  

9.you can't swim, can you?