A new security system was installed. ________, extra guards were hired.

A.As a result

B.Apart from

C.In that case

D.In addition

答案及解析： D

解析：

题意：一个新的安全体系已经设置好了。另外还雇用了额外的门卫。in addition意为"另外"，符合题意。as a result"结果"；

apart from"除了"；

in that case"如果是那样的话"。