A．Their inability to circulate water.
B．Their increased sensitivity to heat.
C．Low reproductive rates.
D．Heavy pollution in the atmosphere.

A．It is damaged by extremely dry weather.
B．It loses water to stronger trees.
C．Insects destroy the tree' s bark.
D．Certain beetles introduce a fungus to the tree.

A．By controlling the carriers of the disease.
B．By growing a stronger kind of the elm.
C．By watering infected elm trees.
D．By cutting down all infected elms.