What's this? It's letter(字母)P. What colour is it? It's green. What's this and what colour is it? It's letter E. It's brown. What's this letter? It's letter N. It's red. P—E—N. Is it“pen”? Yes, it is.

    I have a blue pen in my pencil case(铅笔盒). My pencil case is orange. This is my book. It's my Chinese book. It's in my bag. My bag is black. Look! The bag is on the chair. The chair is yellow. It's my chair. It's nice. I like it.

设函数f（x）=e^x﹣ax，a是常数．

（Ⅰ）若a=1，且曲线y=f（x）的切线l经过坐标原点（0，0），求该切线的方程；

（Ⅱ）讨论f（x）的零点的个数．

如图所示，两平行的虚线间的区域内存在着有界匀强磁场，有一较小的三角形线框abc的ab边与磁场边界平行，现使此线框向右匀速穿过磁场区域，运动过程中始终保持速度方向与ab边垂直．则下列各图中哪一个可以定性地表示线框在上述过程中感应电流随时间变化的规律？（   ）