A lot of business happens over ____ internet.

CONCEPT: English marks a noun with the definite article "the" when it refers to a single, universally recognised entity that speaker and listener both already identify — proper institutions and systems such as "the sun," "the government," or "the internet." Because there is only one such referent shared by everyone, "the" signals "the one we both know," unlike "a/an," which introduces one unspecified member of a larger set, or no article at all, which suits general, non-specific reference to an uncountable idea.

APPLICATION: In "A lot of business happens over ____ internet," the sentence does not describe one of several possible internets — it points to the single, shared global network everyone already knows. That is exactly the situation that calls for the definite article, giving "A lot of business happens over the internet."

CROSS-CHECK — why the other forms don't fit:

• "an" pairs an indefinite article with the network, treating it as one of several unspecified instances rather than the one shared system everyone means.

• "a" carries the same mismatch, and is additionally the wrong article shape before a word that begins with a vowel sound.

• Dropping the article altogether works for abstract mass nouns used in a general sense, but the standard way this specific, well-known network is referred to keeps an article before it.

Result: the sentence takes the definite article — "over the internet."