A：人类的皮肤：黑、白、黄
B：玉米粒的黄、白、红
C：A，B，O 血型
D：生长在阴凉处的小麦秆细、瘦小
— Listen! Someone is knocking at the door. Who can it be?
— It _______ be Tom. He said he would visit me this evening.
