It''s 7:30 in the morning.It''s time to ------------.

It''s 7:30 in the morning.It''s time to ____________.
A.go to bed B.go home C.go to school D.get up
A,B我知道不能选,就是不知道D为什么不能选,要理由,
"Whose bikes are these?" "They are _____."
A.the twins B.the twins'' C.twins D.twin''s
这一题的答案,B与D我不明,就是给点理由.