Most spacecraft are still at little risk of collision with space debris during their operational lifetimes, but given the numbers of new satellites launched each year, the orbital environment in the future is likely to be less _____.
A. crowded
B. invulnerable
C. protected
D. polluted
E. benign
정답은 E인데 benign이 정확히 어떤 뜻으로 해석될 수 있는지 잘모르겠어요! 그리고 B,C,D의 동사들도 왜 안되는지 모르겠어욤,,ㅠㅠ