Ambiguity

A sentence is grammatically ambiguous when it could have more than one meaning. For example:

Which was developing fast, fuel cells or space flights?

What was too heavy, the battery or the solar panel?

Grammatical ambiguities creep in when you have two items followed by a pronoun that could be standing in for either item/person. (A pronoun is a word standing in place of the person or item's name, e.g. I, him, they.)

To avoid grammatical ambiguity, the example sentences should read:

(The sub-clause is now next to its subject, so the meaning is clear.)

(The repetition of 'battery' make the meaning clear.)