请教白勇《GMAT语法全解》87页10，11题

10. Founded in 1983, the magazine increased its circulation more than double since then, and its advertising.

     - 为什么此处应看since then而选择现在完成时，而不看in 1983而选择一般过去时呢？

11. As measured by the Commerce Department, corporate profits peaked in the fourth quater of 1988 and have slipped since then, as many companies have been unable to pass on higher costs.

     - 书上说in the fourth quarter of 1988决定它所修饰动词应用一般过去时，sine then表明其所修饰动词应用现在完成时。从时间顺序看先peak然后才slip，peak用一般过去时，而slip用现在完成时总觉得逻辑关系不是很顺畅，时间顺序可以这样写吗

谢谢咯！

当然是要看就近原则了啊，因为since  then 距离double 最近了啊，是修饰double 的。

对阿，peaked 是一般过去时态。表明是发生在过去，那么slip就发生啊peak以后啊，现在完成时态表明从过去的某个时间开始一直持续到现在，所以用现在完成时态啊。