You needn't have apologized for your absence—________， it's hardly your fault.

A．therefore

B．however

C．meanwhile

D．besides

对进入土壤中的固体养分物质被机械阻留而保存其中的作用叫（）

A.机械吸收

B.物理吸收

C.化学吸收

D.离子交换吸收