z=a+bi (a,b∈ R)，a 实部，b 虚部；对应复平面上的点。
You are eating at Émile's restaurant and the waiter informs you that you have (a) two choices for appetizers: soup or…
两道开胃菜、三道主菜、两道甜品，一共几种吃法？答案是 12，乘法一按就出来。
