求几道英语单选题!

求几道英语单选题!
1.---------dinosaurs suddenly disappeared still remains a mystery
A what B why C that D whether
2-----------is even more important is that as the earth cooled down ,water began to appear on its surface .
A that B which C who D what
3.It was not immediately obvious -------- water was to be fundamental to the development of life .
A that B which C that D 不填
4.His trouble is--------he can find a new job .
A that B where C what D why