( a ) inhibit ovulation and alter the quality of cervical mucus to prevent/ retard entry of sperms
( b ) inhibit ovulation and implantation; alter the quality of cervical mucus to prevent/ retard entry of sperms
( c ) alter the quality of cervical mucus to prevent/ retard entry of sperms
( d ) inhibit implantation as well as alter the quality of cervical mucus to prevent/ retard entry of sperms
