A $若a\equiv b(modm),则m\mid a-b。$

B $若a\equiv b(modm),则a^{2} \equiv b^{2} (modm).$

C $若a\equiv b(modm),c\equiv d(modm),则ac \equiv bd (modm).$

D $若ac \equiv bc(modm),则a\equiv b(modm).$