单选题 ______ Dr. Rossi hired the new assistant, the office has become more organized.

A、 (A) When
B、 (B) Before
C、 (C) While
D、 (D) Since

【正确答案】 D

【答案解析】Since is the correct subordinate conjunction showing cause and effect. Choice (A) shows cause and effect but would need the past tense in the main clause to be parallel. Choice (B) is incorrect because of the present perfect use of have become in the main clause. Choice (C) means although or during.