The enzyme beta-galactosidase acts on lactose to form galactose. in turn, the presence of galactose leads to expression of the enzymes responsible for the metabolism of galactose. in this case, lactose is serving as a carbon source and as a(n):

an inducer

Explanation:

An Enzyme is a protein molecule found in cells which work as biological catalysts.

They speed up chemical reactions in the body, but do not get used up in the process, and can be used over and over again.

In chemistry inducers are molecules that regulates gene expression, and can bind to protein repressors or activators.

Lactose here acts as an inducer because it inactivates the repressor of an operon.

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