— Look! How long _____ like this?

　  — Three weeks! It’s usual here that rain _____ without stopping these days of the year.

A．had it rained; pours

B．has it been raining; pours

C．is it rained; is pouring

D．does it rain; pours

机体吸收外源化学物有哪些途径？各自特点？