16.Mr Chau is the one ______is wearing a grey suit.
A. who
B. which
C. whom
D. he
17.Oxygen is a gas _____ is necessary for all forms of life.
A. what
B. that
C. who
D. it
18. David likes people _____will listen to him when he has a problem.
A. which
B. whom
C. who
D. they