There _____ a printer and six sheets of paper on the desk. [ ]
A. be
B. being
C. is
D. are
无菌包灭菌后有效期为( )
A.1天
B.2天
C.7天
D.14天
设f(x)连续,且,则f(x)=()。
A.2x-x2
B.(2x-x2)e-x
C.(2x-x2)ex
D.2xe-x