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A muscle cell contracts or expands when _____. A. actin and myosin filaments slide over each other B. axons transmit signals to chloroplasts C. xylem and phloem open and close D. cilia project outward from the cytoplasm

Answer :

BeanBaby101

Answer:

I'd say the answer is A.) actin and myosin filaments slide over each other

Explanation:

It's called the "sliding filament theory'

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