Mark the letter A, B, C, or D to indicate the correct response to each of the following exchanges. Two friends Nam and Lan are talking about the topic of global warming.
Nam: What is the biggest environmental problem facing our country? - Lan: ____
A. Too much toxic sewage is dumped at sea.
B. All countries face environmental problems.
C. No one can help protect the environment.
D. Our country is facing the biggest environmental problem.