我国婚姻法明确规定：“直系血亲和三代之内的旁系血亲禁止结婚”，对任何一个人来说在下列血亲关系中属于旁系血亲的是[ ]

A．姨母、姨父

B．祖父、祖母

C．表兄弟、表姐妹

D．外祖父、外祖母

The little boy is dirty from head to foot because he ___ in the mud all morning.        

A．has played

B．is playing

C．has been playing

D．was playing