问题如下：
6. If HiFly and Sky Systems attempt to merge, the increase in the Herfindahl–Hirschman Index (HHI) and the probable action by the Department of Justice and the FTC, respectively, in response to the merger announcement are:
选项：
Increase in the HHI
Probable Response of Department of Justice and FTC
290
To challenge the merger
290
To investigate the merger
400
To challenge the merger
解释：
C is correct.
The pre- and post-merger HHI measures are 1,550 and 1,950, respectively. Not only is the HHI increasing by 400 points, but the industry concentration level also moves from moderately to highly concentrated. The probable action by the regulatory authorities is thus a challenge.
请问什么时候是 investigate the merger，什么时候是 challenge the merger？
