（2011内蒙古乌兰察布，1，3分）4 的平方根是（ ）
A．2
B．16
C．±2
D．±16
答案：C
∵（±2）2=4，∴4的平方根是±2．故选C。
We can't _____________ in spring. [ ]
A. skiing
B. ski
C. skied
The art club is for members only. You can’t go in ____ you are a member.
A．unless
B．because
C．if
D．though
