问题如下：
A distribution with excess kurtosis less than zero is termed:
选项：
A. mesokurtic.
B. platykurtic.
C. leptokurtic.
解释：
B is correct. A platykurtic distribution has excess kurtosis less than zero.
A is incorrect because a normal or other mesokurtic distribution has excess kurtosis equal to zero.
C is incorrect because a leptokurtic distribution has excess kurtosis greater than zero.
