—What _________you_________last week?
—I bought a bag.
A:did; buy
B:did; bought
C:do; buy
D:do; bought
三大合成材料是指 ____、________ . 塑料主要成分是 ____主要性能是 ____型和 ____型.复合材料由 ____ 和 ____组成.
下列哪种食物中含糖较高( )