Which of the following is untrue about the modification of PAM matrices?

• A. At one time, the PAM250 scoring matrix was modified in an attempt to improve the alignment obtained
• B. All scores for matching a particular amino acid were normalized to the same mean and standard deviation, and all amino acid identities were given the same score to provide an equal contribution for each amino acid in a sequence alignment
• C. This took place in 1976
• D. These modifications were included as the default matrices for the GCG sequence alignment programs in versions 8 and earlier and are optional in later versions