单选题 <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14288256_0.png"><br/> A.一定有定义<br/> B.一定无定义<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14288256_0_1.png"><br/> D.可以有定义，也可以无定义