The oceans are the main source of humidity, but plants also pour moisture into the air. In

one day, a five-acre forest can release 20,000 gallons of water, enough to fill an average swimming, pool. A dryer extracts moisture from wet clothes, adding to humidity. Even breathing contributes to this sticky business. Every time we exhale, we expel nearly one pint of moist air into the atmosphere.

Using sophisticated measuring devices, science is learning more and more about the far-reaching and often surprising impact humidity has on all of us.

Two summers ago angry callers phoned American Television and Communications Corp.'s cable-TV operation in northeastern Wisconsin, complaining about fuzzy pictures and poor reception. "What happened," said the chief engineer, "was that the humidity was interfering with our signals." When a blast of dry air invaded the state, the number of complaints dropped sharply.

Humidity plays hob with our mechanical world as well. Water condensation on the playing beads and tapes of videocassette recorders produces a streaky picture. Humidity shortens the life of flashlight and smoke-detector batteries. When the weather gets sticky, the rubber belts that power the fan, air conditioner and alternator under the hood of our cars can get wet and squeak.

Moisture also causes pianos to go out of tune, often in no time flat. At the Wolf Trap Farm Park for the Performing Arts in Vienna, where pianos are tuned twice a day during the summer concert season. Often a tuner stands in the wings, ready to make emergency adjustments during performances.

Humidity speeds the deterioration of treasured family photos and warps priceless antiques. Your home's wooden support beams, doors and window framers absorb extra moisture and expand-swelling up to three percent depending on the wood, its grain and the setting.

Too much moisture promotes blight that attacks potato and green-bean crops—adding to food costs. It also causes rust in wheat, which can affect grain-product prices.

Humidity affects our health, as well. We get more migraine headaches, ulcer attacks, blood clots and skin rashes in hot, humid weather. Since 1987, the Health, Weight and Stress Clinic at Johns Hopkins Hospital in Baltimore has tested over 1700 patients for responses to high humidity. They have reported increased dizziness, stomachaches, chest pains, cramps, and visual disturbances such as double and blurred vision.

The main idea of the passage is about ______.

A．the main source of humidity

B．the impact of humidity on our world and ourselves

C．how humidity affect our life

D．the damage humidity has done to our world