. . . "t, each represents a hydrogen atom, a fluorine atom, an alkyl group having a fluorine atom, a monocyclic or polycyclic cycloalkyl group having a fluorine atom, an alkenyl group having a fluorine atom, an aralkyl group having a fluorine atom, an aryl group having a fluorine atom or an alkoxy group having a fluorine atom, provided that at least one of R19, R20 and R21 is a substituent other than a h" .