Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Smooth muscles and cardiac muscles are not attached to bone. Recall that these types of muscles are under involuntary control. Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. Like skeletal muscles, smooth muscle fibers do contract together, causing the muscle to shorten. Smooth muscles have numerous functions, including the following.


Question: What kind of muscle is responsible for hollow organs contracting?
smooth muscle
Smooth muscle.