1991 AMC 12 Problem 15

No empty chair can have both neighboring chairs empty

Between consecutive occupied chairs there can be at most two empty chairs

The condition says that every empty chair has an occupied neighbor, so no three consecutive chairs may all be empty. Therefore each occupied chair can account for at most itself and the two empty chairs following it, giving $60\le3N$ and $N\ge20.$ Repeating the pattern occupied-empty-empty around the table attains $N=20.$

Thus the correct answer is B.