—Excuse me, sir, visiting hours are over. You leave.
—Pardon me, nurse. I didn't hear the bell.
A:may
B:can
C:must
D:need
某滑轮组改进后提高了机械效率,用这个滑轮组把同一个物体匀速提高到同样的高度,改进后与改进前相比( )
提取玫瑰精油的有关说法中正确的是( )
(文科)若{an}为等差数列,sn是其前n项的和,且S11=22π3,则tana6=( )