By saying "the U.S. and China have opposite problems" (Line 1, Para.1), the author thinks that the saving problems in the U.S. and China are ______.

A．Intellectual challenge.
B．Physical challenge.
C．Social challenge.
D．Economic challenge.

A．Through the living-room window.
B．Through the kitchen window.
C．The door was unclosed.
D．The neighbour gave him the key.