—I'm dead tired. I can't walk any further, Jenny.
—____, Tommy. You can do it.
A:No problem
B:No hurry
C:Come on
D:That's OK
—What ________ they________?
—They were difficult.
草履虫具有比较复杂的结构,这是细胞分裂和分化的结果。
某几何体的三视图如图所示,则该几何体的表面积为( )