他对针灸的益处持怀疑态度。

He's ___dubious about the benefits of acupuncture.

我对于他们对创造一个更美好世界所做的贡献相当怀疑。

I am rather ___dubious about their contribution to a better world.

投资者对这家公司的长期前景仍然存疑。

Investors remain ___dubious about the company's long-term prospects.