Blood cells are produced in which system?
a. circulatory system
b. immune system
c. skeletal system
d. endocrine system

Correct answer is c  - Blood cell are produced in the skeletal system.

Red blood cells are produced on a regular basis in the bone marrow. Bone marrow is a red spongy material found within the cavities of many of the large bones of the body.

Red blood cell start as immature cell in the bone marrow and after about seven days of maturation, they are released into the bloodstream, that is, the circulatory system where they circulate for about 120 days before being replaced with new ones.

the answer is; comparing the frequency of collisions with and without the device

this makes the anti-collision system an independent variable and the collisions a dependent variable. if there are fewer collisions with an anti-collision system that without, then it can be stated that the system is a success. essentially, collisions under an anti-collision system are compared with the frequency of collisions by cars with the human drivers.