Read the following passage:

blue-green light therapy stands alone as perhaps the least intrusive (treatment for sad), and sufferers can purchase special lamps and light bulbs for little more than the cost of a standard desk lamp. the good news is that dozens of researchers have documented the effectiveness of the remedy, which has the same effects as genuine sunshine.
which of the following is the best example of purpose for the passage?

people who make blue lights could charge more money.
the sunlight is unnecessary so we should avoid it.
sad should have more cures.
people should use blue light to feel better.