无黏结预应力束的锚具要求较高，至少应能承受预应力束规定拉力的1.5倍而不超过预期滑动值。( )
参考答案：错
Most people think that English is _____ useful language, but some students think it's _____ unimportant subject. [ ]
A. a, an
B. a, an
C. an, an
D. an, a
裸子植物与被子植物在外形上有哪些区别？
