Hi Everyone!

What do you all think about this problem? I got it wrong and am unsure why.
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Federal authorities involved in the investigation have found the local witnesses are difficult to locate, reticent, and are suspicious of strangers.

(A) the local witnesses are difficult to locate, reticent, and are
(B) local witnesses to be difficult to locate, reticent, and are
(C) that local witnesses are difficult to locate, reticent, and
(D) local witnesses are difficult to locate and reticent, and they are
(E) that local witnesses are difficult to locate and reticent, and they are
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I picked D, but the answer is actually C. But why??