Which of the Following Transactions Would Take Place in the "Underground

Which of the following transactions would take place in the "underground economy"?

A) Elizabeth tells her mother she's going to work but she really goes to see a movie.
B) Paul buys 30 litres of gasoline for $1.29 a liter, not realizing that a fraction of that price goes to the government as tax revenue.
C) John makes chain-link belts and necklaces and sells them at a local flea market for cash to avoid paying taxes.
D) Matt bought an iPad for $499 but decided to sell it on eBay instead of keeping it.
E) Sam works as a consultant on the side and doesn't tell the university he works for.