Read the article and select the best answer. Article: A Nepalese teenager had made a PS 23 solar panel ( ) using human hair. Milan Karki, who is 18 years old and lives in a village, used human hair to replace silicon, which is a common but expensive part of solar panels. By using hair as a replacement, Karki says that solar panels can be produced for around PS 23. Karki got the idea from a Stephen Hawking book, which explained how to create energy from hair. The device that Karki has showed can produce 18w of energy -- plenty to _ a mobile phone. "Half a kilo of hair can be bought for only 16p in Nepal and last a few months; but a pack of batteries would cost 50p and last a few nights," according to The Daily Mail. The solar panel is easy to service as the hair is easy to replace. Karki has now sent out several devices to other places near his home for testing. He said, "First I wanted to provide energy for my home, then my village. Now I am thinking for the whole world." Question: What is the passage mainly about? Options: A: Making a mobile phone with human hair B: Making a mobile phone with a new idea. C: Making a solar panel with silicon. D: Making a solar panel with human hair.
D