Evidence has been found through years of study______ children’s early sleeping problem likely to continue when they grow up.

A．why

B．how

C．whether

D．that

下列选项中，哪一项为甘草外皮的性状特征（）

A、外皮常松紧不等

B、栓皮薄，多破裂向外卷曲

C、栓皮易剥落而露出淡黄色的皮部及纤维

D、有纵皱纹及横长皮孔

E、有时可见残存的淡棕色外皮