[单项选择题] 阅读下列程序段，不合法的行是

[单项选择题] 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

B