A．To let the children correct their own papers.
B．To let the children measure their own understanding.
C．To exam children and mark their papers.
D．To help children to find the way to get right answers.

A．Ozone is not always associated with acid rain.
B．Cold weather would make the effects of pollution worse.
C．Many industries are unwilling to accept responsibility for air pollution.
D．Plants in warm weather are dying faster.