Choose the correct answer

13. Did your children enjoy ___ in the sea?

A. swimming

B. swim    

C. to swim

D. swam

14. "When is Mr Ba planning to retire?

- He ___ for the company for thirty years now".

A. works

B. is working

C. will work

D. has been working

15. Flowers should ___ in warm places.

A. be keep

B. kept

C. be kept

D. be keeping

16. They went on walking ___ it started to rain.

A. though

B. because

C. but

D. despite