1吨废纸可造出（）千克好纸。

It is clear that a computer works ______ faster than an abacus .

A．a lot

B．a lot of

C．little

D．very

—Can I watch the football match on TV tonight, Mum?

—_____ you finish all the homework first.[ ]

A. If only  

B. Only if

C. Even when  

D. Not unless

绘画艺术

已知y^*=e^xsinx+excosx+e^2x是二阶常系数线性微分方程y"+ay’+by=ce^2x的一个特解，试确定常数a，b，c的值，并求此方程的通解．

阐述垂体促性腺激素的作用。