A．The ground is too hard for planting.
B．Transportation is expensive.
C．There has been too much rain in the Yangtze River valley this year.
D．The planters are experimenting with new crops.

A．Wearing arm weights while you are swimming.
B．Jogging vigorously in one place for a long time.
C．Using bicycles that require you to use both your arms and legs.
D．Walking slowly while swinging your arms back and forth.