Bell peppers were originally small and green, they became large and multicolored only during the twentieth century.

Correct sentence- Bell peppers were originally small and green, becoming large and multicolored only during the twentieth century.

When questions involve comma splices, answers that contain -ing (participles) are considerably more likely to be correct because they create dependent clauses and prevent commas from separating two full sentences.

Cucumbers are used in many different types of cooking, and are grown around the world in both fields and greenhouses.

Correct sentence- Cucumbers are used in many different types of cooking and are grown around the world in both fields and greenhouses.

When the subject is the same in both clauses and is not repeated in the second clause, do not use a comma.

I like that princess gown, however, it comes only in one color.

Correct sentence- I like that princess gown. However, it comes only in one color.
I like that princess gown; however, it comes only in one color.

Certain transitions (formally known as conjunctive adverbs) are considered ""strong"" enough to begin a sentence. The ones on which you are most likely to be tested include however, therefore, thus, consequently, moreover, and nevertheless. When used to begin a clause, these transitions should always follow a period or semicolon, never a comma.