A beam of unpolarised light of intensity $I_{_0}$ is passed through a polaroid A and then through another polaroid B which is oriented so that its principal plane makes an angle of $45^{\circ}$ relative to that of A. The intensity of the emergent light is:
( A ) $I_{_0}/4$
( B ) $I_{_0}/2$
( C ) $I_{_0}/8$
( D ) $I_{_0}$