下面哪几项属于重要空白凭证？（）
A.存单
B.支票
C.单位定期存款证实书
D.收贷收息凭证
参考答案：A, B, C, D
将用15N标记的一个双链DNA分子，加入到含有14N标记的原料的培养基中，连续复制三代，其后代中含有14N和15N的DNA分子之比为[ ]
A．7：1
B．8：1
C．4：1
D．8：0
He _____ articles for our wall-newspaper these three years, and he _____ about forty articles.
A．has been writing； has written
B．has been writing； wrote
C．is writing； has been writing
D．has written； has written
