他明天能否出席会议还很难说。

It is ___doubtful whether he'll be able to attend the meeting tomorrow.

丢失的钥匙不太可能找回来了。

It was ___doubtful that the lost keys would be recovered.

声称成功完全取决于天赋，这不太可信。

It is ___doubtful to claim that success depends solely on innate talent.