Although medical science________ control over several dangerous diseases, what worries us is that some of them are returning.

A．achieved

B．has achieved

C．will achieve

D．had achieved

（）使用系统的功能数量来测量其规模，它以一个标准的单位来度量软件产品的功能，与实现产品所使用的语言和技术没有关系的。