钩藤降压作用的主要成分是（）
A.钩藤碱
B.多肽物质
C.生物碱
D.琥珀酸
E.挥发油
参考答案：A
--- I ____ so busily recently that I ____ no time to help you with your maths.
---That’s OK. I can manage it by myself.
A．had been working; had had
B．have been working; had
C．have been working; have
D．am working; will have
目前广泛应用的人类生育控制技术不包括（）。
A.避孕
B.人工流产
C.绝育
D.产前优生学诊断
E.克隆
