1She was doubtful about the new business plan, so she asked for more details.
2Are you doubtful about your ability to finish the project on time?
3He was doubtful that the weather forecast was accurate, so he checked another source.
4If you are doubtful about the safety of the product, do not use it.
5There are still many doubtful areas in the new policy that need to be clarified before it can be implemented.