This is a technical question in US patent law, and I need a precise
answer.
Imagine Application A is a CIP of Application B, and B is a CIP of
Application C.
I want to file a new application X, as a CIP of A, but I do not want
to have
publication of X be based on 18 months from the filing date of C (as
would normally be the case -- the expected date of any US regular
patent application filed in the last few years is 18 months from its
earliest priority date).

Can I make Application X only claim the priority of the filing date of
A, but not claim priority from the filing dates of B and C?  In this
way, I would hope to make the publication date of Application X be 18
months from the filing date of Application ***A***.

I need a citation from MPEP (the Manual of Patent Examining Practice)
or from US patent law (35 USC xxx).
Please DO NOT just send me the passages from MPEP which generally
discuss the topic of US publication practice post-November 2000
(unless you are doing it as warm-up to presenting your answer).  This
is a very specific question -- either it is possible to "cut off" the
priority claim to only the last in the string of CIP's, or it is not
possible.

Very Intersting question

""Subtitle E of the American Inventors Protection Act of 1999 ("the
AIPA"), titled Domestic Publication of Patent Applicants Published
Abroad, mandates that, with certain specified exceptions, U.S. patent
applications "shall be published promptly" 18 months from their
earliest priority date.""

Example
Parent Patent Application filed -- June 30, 1998
CIP of Parent Fled - June 30, 2001
Publication of CIP - Approx.  14 weeks after CIP filing

However, if you are not going to file for a foreign patent seeking
priority of the CIP, you can submit a non-publication request
concurrently with the filed application.

"Therefore, applicants who prefer non-publication and have not
previously filed a foreign (or international) application that is
subject to 18 month publication should consider filing non-publication
requests at the time of filing, even if they have not yet decided
whether to file outside the U.S. If the applicant subsequently decides
to file a foreign (or international) application that will publish
after 18 months, however, he or she must notify the USPTO within 45
days after the date of such filing. If the USPTO is not timely
notified of the foreign filing, the U.S. application will be
abandoned, subject to revival if the delay in giving notice was
unintentional. Even if the applicant does not file abroad, he or she
may rescind the non-publication request at any time."

No matter what you decide, DO NOT give up your priority of the
earliest filed application.  You will then get a 102(e) rejection due
to the earlier filed application.

If you do not want pre-grant publication and the original Parent
application was filed in a foreign country, there is always the option
of filing the New Matter as a Parent Application without seeking
priority.  However, again you risk a 102 rejection, so be careful.