—Mark broke his leg when he was playing football .

—         was that ?

A．Since when

B．Since when ago

C．How long

D．How long ago

黄土高原处于我国多种地理单元的过渡地带，下列叙述不正确的是[ ]

A．从平原向山地、高原过渡

B．从湿润地区向半湿润地区过渡

C．从沿海向内陆过渡

D．从森林农业向草原牧业过渡