直肠癌的早期症状是（）
A.大便变细
B.排便习惯改变
C.便血
D.脓血便
E.排便时疼痛
参考答案：B
Tom's mother is good at cooking and she knows _____ to make food. [ ]
A. what
B. how
C. why
钢坯加热时必须满足什么要求？
