553.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
554.The construction of the two new railway lines _______ by now.
A.has completed
B.have completed
C.have been completed
D.has been completed
553.答案是B;根据语境，此题应采用现在完成时。
554.答案是D;by now与现在完成时连用；此句主语是the construction,故谓语动词应用单数形式。
