Describe a contact mine.

A contact mine is defined as a type of explosive device that detonates when it comes into physical contact with a vessel. This characteristic is essential because contact mines are typically placed in water and are designed to target ships or submarines directly. Upon contact, they trigger an explosive reaction that can cause significant damage to the hull of the vessel.

The other choices describe different types of mines or trigger mechanisms. The first option refers to pressure mines, which activate when weight is applied, usually suitable for land-based scenarios. The third options involve mines that react to environmental factors, like underwater currents, and do not fit the fundamental principle of contact detonation. The last choice pertains to timed or self-destructing mines that operate independently of the physical contact criteria. Understanding these distinctions helps clarify why choice B accurately captures the essence of a contact mine.