watever it is
the one letter single is always a Huge Hit and the highest selling in that year ony for ayu :p
like
in 1999 - A -1.6 mill - highest in this year
in 2001 - M - 1.3 mill - highest in this year
in 2002 - }{ - 1.1 mill - the only million seller single of the year - highest in this year
in 2003 - & - 650 k - highest in this year
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and also SEASONS could have been the S - Single ,,, but S is not in her name :p
my theory is lame