Today's English lesson

From the American Heritage Book of English Usage:

Or

When all the elements in a series connected by or are singular, the verb they govern is singular:

Tom or Jack is coming.

Beer, ale, or wine is included in the charge.

When all the elements are plural, the verb is plural. When the elements do not agree in number, some people say that the verb should agree in number with the nearest element:

Tom or his sisters are coming.

The girls or their brother is coming.

Cold symptoms or a headache is the usual first sign.

But others object that these constructions are inherently illogical and that the only solution is to revise the sentence to avoid the problem of agreement:

Either Tom is coming or his sisters are.

The first sign is usually cold symptoms or a headache.