A few years ago, in their search for ways

题目：

A few years ago, in their search for ways to sell more goods, advertising men hit on a new and controversial gimmick. It is a silent, invisible commercial that, the ad men claim, can be rushed past the consumer’s conscious mind and planted in his subconscious—and without the consumer’s knowledge.

Developed by James Vicary, a research man who studies what makes people buy, this technique relies on the psychological principle of subliminal perception. Scientists tell us that many of the sights coming to or eyes are not consciously "seen" . We select only a few for conscious "seeing" and ignore the rest. Actually the discarded impressions are recorded in the brain though they are below the threshold of consciousness.

There’s little doubt in Vicary’s mind as to the subliminal ad’s effectiveness. His proof can be summed up in just two words: sales increase.

In an unidentified movie house not so long ago, unknown audiences saw a curious film program. At the same time, on the same screen on which the film hero was courting the heroine a subliminal projector was flashing its invisible commercials.

"Get popcorn," ordered the commercial for a reported one three-thousandths of a second every five seconds. It announced "Coca-Cola" at the same speed and frequency to other audiences. At the end of a six weeks trial, popcorn sales had gone up 57 percent, Coke sales 18 percent.

Experimental Films. Inc, says the technique is not new. It began research on subliminal perception in 1954. Experimental Films stresses that its equipment was designed for helping problematic students and treating the mentally ill. At NYU two doctors showed twenty women the projected image of an expressionless face. They told the subjects to watch the face for some change of expression. Then they flashed the word angry on the screen at subliminal speeds. Now the women thought the face looked unpleasant. When the word happy was flashed on the screen instead, the subjects thought the woman’s facial expression looked much more pleasant.

Subliminal techniques, its promoters believe, are good for more than selling popcorn. Perhaps the process can even be used to sell political candidates, by leaving a favorable impression of the candidate in the minds of the electorates subliminally.

How convincing are these invisible commercials Skeptical psychologists answer that they aren’t anywhere near as effective as the ad men would like to think they are. Nothing has been proven yet scientifically, says a prominent research man.

Subliminal ads are invisible because they are shown very ().

A. fast

B. naturally

C. often

D. vaguely