My mother is ill.So I ______to look after my little brother.
A：must
B：may
C：have to
D：should
①“x=1”是“x2﹣3x+2=0”的必要不充分条件
②命题“∀x∈R，cosx≤1”的否定是“∃x0∈R，cosx0≥1”
③若一个命题的逆命题为真，则它的否命题一定为真．
