If the sentence’s subject has two or more nouns or pronouns connected by “and,” use the form of the verb. irregular plural regular singular

If the sentence's subject has two or more nouns or pronouns connected by "and" you would use the plural form of the verb

The mistake which Daniel Barringer made was that:

He thought that iron must be found in a meteorite crater.

After studying the Arizona crater, he was convinced that the cause of the appearance of the crater was the huge iron meteor he had found near the crater’s rim.