The main idea of the passage is that _________.
A．the possible exit of the United States could mark the end of the Kyoto Protocol
B．the terms of the Kyoto Protocol are overly ambitious, and thus inapplicable in reality
C．in order for the Kyoto Protocol to succeed, it must be reviewed and possibly modified
D．in light of its unequal treatment of under-developed nations, the Kyoto Protocol is doomed to failure