Which of the following situations represent equilibrium?

(a) Migratory birds fly north in summer and south in winter.

(b) In a grocery store, some carts are kept inside and some outside. Customers bring carts out to their cars, while store clerks bring carts in to replace those taken out.

(c) In a tug o’ war, a ribbon tied to the center of the rope moves back and forth until one side loses, after which the ribbon goes all the way to the winner’s side.

(d) As a stew is cooking, water in the stew vaporizes, and the vapor condenses on the lid to droplets that drip into the stew.

A reaction at equilibrium can be stated as forward reaction is equal to backward reaction. Given situation is migratory birds fly north in summer and south in winter. This situation is in equilibrium because north and south direction are in opposite way. Migratory birds fly north is equals with birds fly south in winter.

Situation 2 is also at equilibrium because when customers purchased the items from shop, store clerks replace those taken items. That means both sides the items are equal and there is no decrease or increase in items happened. So, this situation is in equilibrium.

In 3^rd situation the rope is moving front and coming back with respect to force applied by participants. But at last, the rope will move towards the more force applied side. So, this cant be taken as s situation for equilibrium.

Vaporization of water molecule from cooked items is a common situation. The time at lid open, we can see vaporized water molecules are stick to it are falling at the cooked items. This means both sides water molecule concentration is equal and so, this situation is in equilibrium.