Sentence Correction

From OG 12 SC 69:
Once they had seen the report from the medical
examiner, the investigators did not doubt whether
the body recovered from the river was the man who
had attempted to escape from the state prison.
(A) did not doubt whether the body recovered from
the river was
(B) have no doubt whether the body recovered
from the river was
(C) had not doubted that the body recovered from
the river was
(D) have no doubt whether the body recovered
from the river was that of
(E) had no doubt that the body recovered from the
river was that of

Can someone explain how to pick between D and E?

[spoiler]
My source of uncertainty is in Split #1. Can someone explain please, preferably with examples?

Split 1) HAD no doubt versus HAVE no doubt
Why is HAD necessary in the tense if the explanation provided says "Once X HAD HAPPENED, then Y HAPPENED"? Y's verb does not have "HAD". I am unclear on this point.

Split 2) was versus that of
That of is required because body needs to be compared to the body of the man and not the man itself
[/spoiler]

[spoiler]
OA is E
Explanation:
As it is used in the opening clause, the past
perfect verb tense indicates that an action in the
past was completed before another action in the
past: Once x had happened, then y happened. To
maintain proper verb sequence, the verb in the
main clause must therefore be in the simple past
tense. When doubt is used in a negative context
such as there is no doubt or he does not doubt, it
should be followed by that. A dead body cannot be
a man; the body is that of a man.

A Whether should be that; the body is that of a
man.
B Incorrect use of present verb have instead of
had; whether should be that; the body is that
of a man.
C Incorrect use of past-perfect verb had not
doubted instead of did not doubt; the body is
that of a man.
D Incorrect use of present verb have instead of
had; whether should be that.
E Correct. In this sentence, had no doubt is the
proper tense and is correctly followed by
that; that of is used to refer to the body of a
man.
Th e correct answer is E.
[/spoiler]

I see my error now.
Given the fact that I have seen the body, I have no doubt it is the prisoner
I HAVE SEEN = past perfect
I HAVE = simple past

Given the fact that I had seen the body, I had no doubt it was the prisoner
I HAD SEEN = past perfect
I HAD = simple past (I erred here thinking that HAD was past perfect but no linking/helping verb follows HAD, so it becomes SIMPLE PAST by default)

Hence E is preferred over D.

Yes, HAD is both a helping verb in Past Perfect and a verb in Past Simple, as in:

I had a dog.
I had no doubt.

gkumar wrote: I HAD SEEN = past perfect
I HAD = simple past (I erred here thinking that HAD was past perfect but no linking/helping verb follows HAD, so it becomes SIMPLE PAST by default)

Hence E is preferred over D.

Kumar,

Just to clarify that had followed by a past participle is a past perfect, so Had Seen is past particple, however had with out a apst participle is a simple past, so Had no doubt is simple past.

Please confirm.

Vineet

Yes I agree, Vineet!