After walking on the zigzag path to the mountain top for a few hours, the tourists gathered under trees ________ shade and ________ down to have a rest.

A．provided; sit

B．providing; sitting

C．providing; sat

D．provided; sat

蓄积作用是（）

A.化学物质发生慢性中毒的基础

B.分为物质蓄积和功能蓄积

C.是化学毒物分布的一种特殊形式

D.多来自小剂量的长期接触

E.以上均正确