A：组成不同生物细胞的化学元素种类大体相同
B：组成细胞的最基本元素是C
C：同一生物体的不同细胞内，各种化学元素的含量相同
D：组成生物体细胞的元素，在无机自然界中都能找到
— There's somebody at the door. Who ____ it be? Is it the postman?—No. It ______ be him. It's just seven o'clock. It's too early.
