B.
11. Is this the cheapest shirt do you have?
12. She can play the guiter very well.
13. What does your son weigh?
14. Ha's father is a careful and safe cyclist.
15. The cost of living here is expensive.
16. The headache prevented Mr. Cuong from enjoying the meal. 
17. My mother will be in 15 minutes and so does my father. 
18. Why don't we eat the pineapples after lunch?
19. Both Ba and his close friend can cook very well.
20. What is the weight of this pig?