Directions: Read the following passage and answer the question that follows.

Passage

The explanation of the mystery was really simple enough. The bewildered boys had been following a scientist who had just discovered how to make the human body transparent.

Griffin, the scientist, had carried out some experiment after experiment to prove that the human body could become invisible. Finally when he swallowed certain rare drugs and his body became as transparent as a sheet of glass. Griffin was also a lawless person. He once set fire to his landlord's house. To get away without being seen he had to remove his clothes. Then he became a homeless wanderer, without clothes, without money but quite invisible until he happened to step in some mud and left footprints as he walked. The bewildered boys had been following a scientist… How were the boys able to follow the scientist when he was not visible?