单选题：给定如下代码，哪个表达式是错误的？

给定如下代码，哪个表达式是错误的？ @[D](2)
```
class C1 {}
class C2 extends C1 { }
class C3 extends C2 { }
class C4 extends C1 {}

C1 c1 = new C1();
C2 c2 = new C2();
C3 c3 = new C3();
C4 c4 = new C4();
```
A. c1是C1是实例
B. c2是C1是实例
C. c3是C1是实例
D. c4是C2是实例




A.c1是C1是实例
B.c2是C1是实例
C.c3是C1是实例
D.c4是C2是实例


答案：D