--- Mum, could I play with my classmates for a while after I finish my homework?

--- _______. But you must come back before supper.

A．Yes, you can

B．Yes, you could

C．No, you can’t

D．No, you couldn’t

随着现代化学的发展，出现了各种分析和测试物质结构、跟踪化学反应过程的技术。下列不属于现代化学研究重要手段的是[ ]

A.X-射线    

B.天平    

C.质谱    

D.核磁共振