In a nuclear power plant, the nuclear reaction is kept from going critical by keeping the rate of reaction safe. how do the control rods figure into this?

a. the controls rods absorb the excess heat produced in the reaction.
b. the controls rods absorb excess neutrons, keeping them from causing too many uranium atoms to split too quickly.
c. the control rods provide additional neutrons to keep the reaction going.
d. the control rods absorb excess uranium atoms so that the reaction doesn't happen too quickly.

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?