Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Actin filaments are part of the contractile machinery in smooth muscle cells of eukaryotic organisms. Which organelle is responsible for contractile movement in these cells?
Smooth endoplasmic reticulum (incorrect answer)