Sentence Correction

A patient accusing a doctor of malpractice will find it difficult to prove damage if there is a lack of some other doctor to testify about proper medical procedures.

(A) if there is a lack of some other doctor to testify
(B) unless there will be another doctor to testify
(C) without another doctor's testimony
(D) should there be no testimony from some other doctor
(E) lacking another doctor to testify

Answer C

I eliminated B because it sounds wrong.
I don't know why it is wrong.

Official GMAT Verbal Guide edition 15 explanation:

B. The construction y will happen unless x happens first requires the present tense following the unless clause, rather than future tense used here

I didn't understand the explanation.

Can someone please explain it?
Thanks!

An unless-clause requires the present tense.
unless = if not.
John will not attend unless Mary attends.
Conveyed meaning:
IF Mary does NOT attend, John will not attend.

B: unless there will be another doctor
unless + FUTURE TENSE is always wrong.
Eliminate B.