We are conducting training to devise strategies against hypothetical enemies.

The enemy general is approaching our camp.

Enemy troops are approaching our position.

He experienced his first encounter with the enemy on the battlefield.

He encountered the enemy on the battlefield.

The signal from the cell phone relay-station in this area is weak.

He likes to take a walk in the park at dusk.

Linear programming is a part of operations research and is used to find the optimal solution.

Paying the urban planning tax is a citizen's duty.

The United Nations Environment Programme is providing solutions to global environmental problems.