What encourages a response be it internal or external of the body

Change in external environment is called stimuli. This stimuli encourage the response.

Explanation:

Response evoke internally when the stimuli is detected. Actually the internal environment fluctuates with external environment. Change in external environment try to bring out change in internal environment so to maintain Homeostasis is carried out.

i think it would be b. tissue because an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation: