—Do you regret bringing up your opinion at the meeting?

—No.________, I think it is very necessary. [ ]

A. On the contrary  

B. First of all

C. On the other hand  

D. In a word

下列事例中，能通过食物链引起生态危机的是[ ]

A．酸雨

B．汞、镉等有毒物质的积累

C．温室效应

D．臭氧减少，臭氧层出现空洞