----Which is _______ to understand, this book or that book?

---- I think both of them are quite easy.

A．difficult

B．more difficult

C．the most difficult

D．much too difficult

男，54岁，反复脓血便半年，每天3～ 4次，在当地曾按“痢疾”治疗无明显效果。近一月出现腹胀，伴阵发性腹痛。查体：消瘦，腹稍胀，软，下腹轻压痛，右下腹可扪及一肿块，质较硬，尚可活动。

首选的辅助检查

A．大便细菌培养

B．CT

C．钡餐检查

D．纤维结肠镜检查

E．B超