A:can’t see
B:can’t have see
C:must see
D:mustn’t have see
①若a,b,c∈R,则“ac2>bc2”是“a>b”成立的充分不必要条件;②命题“∃x0∈R, x02 +x0+1<0”的否定是“∀x∈R,x2+x+1≥0”;③命题“若|x|≥2,则x≥2或x≤-2”的否命题是“若|x|<2,则-2<x<2”;④函数f(x)=ln x+x- 32 在区间(1,2)上有且仅有一个零点.
如图,在△ABC中,∠ACB=90°,DE垂直平分AB,垂足为D,如果∠A=30°,AB=63cm,那么CE等于( )
The Greens ________live in the south of Asia . They don't want to move to the north because they ________ the cold weather.