The phrase "become doubtful" is a grammatically correct part of a sentence and is commonly used in written English.
It is typically used to describe a situation where something or someone is no longer certain or reliable. Example: As the evidence continued to pile up, the credibility of the witness's testimony started to become doubtful.
However, in the context of climate change, it is possible that these assumptions would no longer hold and the results of conventional frequency analysis would become doubtful.
Phyllis and Bessie declare Jacob is the returned spirit of the late Bishop, but their followers are beginning to become doubtful.
That would be true unless the bank believed it had become doubtful that it would be able to collect all the money it was owed.
But many governments have become doubtful of their ability to cope with urbanisation on such an enormous scale; some have concluded that they ought to slow the process down in order to minimise social upheaval.
When bonds are present between two elements that differ little in electronegativity, however, oxidation-state assignments become doubtful, and the distinction between redox and nonredox processes is not evident.
If nothing has been agreed, the seller, although no longer the owner, may refuse to deliver or stop the goods en route to the buyer if the latter's solvency has become doubtful after conclusion of the contract.
The topic identification algorithm's performance becomes doubtful in various cases.
The public becomes doubtful and the media, so often looking for controversy and conflict, has been a conduit for the fossil fuelled messages.
Eddie becomes doubtful about a group he has previously followed unquestioningly, much to the concern of his still-devout wife Sarah Michelle Monaghann).
Macchione was not immediately available to comment on why he became doubtful of the cause of death, though Vacchiano says his colleague's change of opinion struck him as strange, as did the judge's remarks on suicide.
Jo joins in with Rufus to torture the supposed "demon" out of a captured Sam with holy water and salt, but becomes doubtful when no demonic effects are present on Sam.
