单选题 ______ flexible work time was introduced, productivity levels have improved by almost five percent.
A. As
B. Before
C. Since
D. While

【正确答案】 C

【答案解析】Since(自从)表示一段时间的起点，主句用现在完成时。选项(A)As(当……时)、选项(B)Before(在……以前)和选项(D)While(和……同时)均不正确。