在空间直角坐标系Oxyz中，点P（2cosx+1，2cos2x+2，0），Q（cosx，-1，3），其中x∈[0，π]，若直线OP⊥直线OQ，求x的值．

I had just stepped out of the bathroom and was busy drying myself with a towel ________ I heard the

steps. [      ]

A. while  

B. when

C. since  

D. after