问题详情

阅读下列程序段，不合法的行是______。 class Computer{} class Monitor extends Computer{} class HardDrive extends Computer { public void test() { Monitor a = new Monitor(); Computer b = new Monitor(); HardDrive c = (HardDrive) a; Monitor d = b; } } ①Monitor a = new Monitor(); ②Computer b = new Monitor(); ③HardDrive c = (HardDfive)a; ④Monitor d = b;
A．①②③
B．③④
C．①③④
D．②④请帮忙给出正确答案和分析，谢谢！