Astudent wants to test the effect of nitrogen rich fertilizers on the growth of rose bushes. which of the following is a variable that must be kept constant?

growth of the rose bush
amount of water the rose bush receives
the use of nitrogen rich fertilizer

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.

answer; nociceptive pain;