Drugs that break down a psychological barrier against doing other drugs are known

The answer to this is B gateway drugs as they act as a gateway to use other drugs

The right answer is (B) Gateway Drugs

Explanation:

Gateway drugs are those chemicals or substances that, when consumed for longer time, give way to harder and more dangerous drugs. These dangerous substances, such as nicotine or alcohol are believed to open the door to drugs such as meth, heroin and cocaine, which can lead to addiction and serious harm to human health. Marijuana, cigarettes and alcohol are the examples of gateway drugs.

In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation at right shows a one-dimensional longitudinal plane wave propagating down a tube. The particles do not move down the tube with the wave; they simply oscillate back and forth about their individual equilibrium positions. Pick a single particle and watch its motion. The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right.The second animation at right shows the difference between the oscillatory motion of individual particles and the propagation of the wave through the medium. The animation also identifies the regions of compression and rarefaction.The P Waves(primary waves) in an earthquake are examples of Longitudinal waves. The P waves travel with the fastest velocity and are the first to arrive. 

Reference: http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

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