单项选择题 Students should be encouraged to _____ different approaches to problems.

判断题 酚醛树脂耐酸腐蚀的性能较环氧树脂、聚酯树脂好，特别是对浓度为50%以下的硫酸和任何浓度盐酸的腐蚀表现出更突出的稳定性，由于发生氧化及降解反应，因此酚醛树脂不耐浓硫酸及硝酸等氧化性酸的腐蚀。

单项选择题 He’s ____ as a “bellyacher”-- he always complaining about something. （ ） A.who is knownB.whom is knownC.what is known D.which is known