What makes a source credible is it has backed up information. Basically, it has to have supporting details so you know its true.
The Gothic element there is the horror that ensues after he hears the tapping. The character is alone during a spooky, dreary night, nobody found anywhere, and suddenly there is a mysterious tapping by someone, and it turns out that no-one tapped, which means either madness or ghosts, that are either way both elements of Gothic writing.
A popular model used to illustrate the concept of opportunity cost is called a. the production possibilities frontier.
Well he did increase the wage from an eight-hour day for his workers to $5
