What is a "spike" in Agile/Scrum terminology?

A A time-boxed research or investigation task undertaken to answer a question or gather information needed to estimate or implement a future user story, rather than to deliver a shippable feature directly B A spike refers to a sudden, permanent increase in a team's velocity that never decreases C It is a term used only to describe a server crash D A spike means adding ten times more developers to a team overnight