(a) a group selected from the group consisting of ##STR1## in which R1, R3 and R5 each is a hydrogen atom or a methyl group, R2 is a hydrogen atom, a chlorine atom, a bromine atom or a methyl group, R4 is a hydrogen atom or an alkyl group containing 1 to 5 carbon atoms and R6 is a methyl group or an ethyl group,