— did you do last weekend?
— I for the English test.
A:What; study
B:What; studied
C:Where; studied
D:How; studied
已知等差数列{an}中共有18项,其中奇数项之和为11,偶数项之和为29,则其公差为( )
①它的沸点低
②它的密度比空气大
③它难溶于水
④它与空气中的成分不反应.