If in doubt as to the true water level when a boiler is being oil-fired, you should ().
We don’t doubt _____ he can do a good job of it.
I doubt whether the Chinese Football Team can win the game this time.()
If in any doubt as to the pilot’s actions or intentions,the officer in charge of the navigational watch shall().
When a vessel is in any doubt as to whether such a situation exists she shall()that it does exist and act accordingly.