passive voice - "hard to distinguish" or "hard to be distinguished"?

Here is a phrase (slightly modified from the original) that I'd like to discuss.

A) targeting small structures that are hard to distinguish

I have no doubt that this will convey what it means, but I think that the following is more "authentic" in terms of grammar:

B) targeting small structures that are hard to be distinguished

Would you agree or not? Can you explain why grammatically?

I tried to explain why I think B is better, but I'm not sure now.

I think the problem can be simplified to the difference between the following examples:

C) small structures are hard to distinguish

D) small structures are hard to be distinguished

It's about what is the subject for the verb "distinguish". Please help!