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

答案及解析： B

解析：

现在完成时表示过去发生的事对现在造成的结果，表示动作已经完成。根据所提供的情景“what worries us that some of them are returning”可判断出一些危险的疾病已经得到了控制，但有它们中的一些疾病又开始传播。