i working on something and i just am completely PUZZLED, its probably something obvious

Thinking rhat you will go for a swim , you approach a swimming pool and submerge your toes into the water. the water is very cold, which promts you to reflexisively to withdraw your foot. in terms of a feedback circuit, in this case the muscles that moved in order to allow you to withdraw your foot quickly can be thought of as which of the following?

i tried to do these but ugh i ahte word problems, same with math ones
i tried to guess/understand them

a. the receptor…noo
b. the control center….maybe? but why?
c. the effector…nah
d. the signaler….yes?
e. the sensor..noo…makes too much sense