Question

how do smooth er control muscle contraction and signal transduction?
by firing nerve signs
by releasing calcium
by flushing potassium

Explanation:

In muscle cells, the smooth endoplasmic reticulum (smooth ER, also called sarcoplasmic reticulum in muscle) stores and releases calcium ions ($Ca^{2+}$). For muscle contraction, the release of calcium from the smooth ER (sarcoplasmic reticulum) into the cytoplasm triggers the interaction between actin and myosin filaments. In signal transduction, calcium ions act as second messengers. Nerve signals are fired by neurons, not smooth ER. Potassium flushing is not the mechanism by which smooth ER controls muscle contraction or signal transduction. So the correct option is the one about releasing calcium.

Answer:

B. by releasing calcium (assuming the options are labeled A, B, C with A: by firing nerve signs, B: by releasing calcium, C: by flushing potassium)