行星际介质
参考答案：
气体和尘埃粒子稀疏分布于行星间。
—I regret _____ you that Mary was badly injured in the accident.
—I can hardly believe my eyes. She is always so careful.
A．informing
B．having informed
C．to inform
D．to have informed
※ [16～18] .
(4 )
A．.
B．.
C． .
D． .
