橡胶按其来源可分为天然橡胶和（）两大类。
参考答案：合成橡胶
Susan usually ________ her homework at 7:00 in the evening.
A．goes
B．does
C．do
D．has
叙述学习建造师的法律责任和提升职业道德素养的重要性？
