Question

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 1

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

Step-by-step explanation:

It's called the "sliding filament theory'