In Which of the Following Explanations Is the Explanandum Underlined

In which of the following explanations is the explanandum underlined?

A) I ironed my shirts again because they still looked wrinkly.
B) Brandy is selling her car to Will, since it is not big enough for two car seats.
C) Because of a belief that it is bad for birds, people don't throw rice at most weddings anymore.
D) Due to the freezing weather, it is a good idea to wear two layers under your coat.
E) Since the 18th was a Monday, the 31st will be a Sunday.