increase by 为什么这里是+不是*？

问题如下：

4. Based on Exhibits 1 and 2, Candidate C’s forecast for cost of sales in 2013 is closest to:

选项：

$18.3 million.

$18.9 million.

$19.3 million.

解释：

C is correct.

Candidate C assumes that the 2013 gross margin will increase by 20 bps from 2012 and that net sales will grow at 50 bps slower than nominal GDP (nominal GDP = Real GDP + Inflation = 1.6% + 2.0% = 3.6%). Accordingly, the 2013 forecasted costs of sales are $19.27 million, rounded to $19.3 million.