The children cheered up when they saw hundreds of colorful balloons ______ slowly into the sky.

A．America's machine-tool industry was losing its market.
B．America' s machine-tool industry was facing strong competition.
C．America's machine-tool industry was losing its position.
D．America's machine-tool industry was in danger.

A．It is human nature to shift between self-doubt and blind pride.
B．Intense competition may contribute to economic progress.
C．The revival of the economy depends on international cooperation.
D．A long history of success may pave the way for further development.

A．Pollution -- the Necessary Evil
B．The Causes of Pollution
C．Pollution and Agriculture
D．Anti-pollution Needs Your Efforts