I admit that all these problems are difficult I don’t agree that they can’t be solved.A.

问题补充：

I admit that all these problems are difficult, I don’t agree that they can’t be solved.

A. Unless

B. While

C. Until

D. Since

答案：

A. Unless

(答案→)B. While

C. Until

D. Since

解析： 词义辨析。Unless除非……否则；While尽管；Until直到；Since

自从，因为，由于。句意：尽管我承认这些问题都很难，但是我不认为它们不能被解决。所以选B。