Use the Following Information When Answering the Corresponding Question(s)

Use the following information when answering the corresponding question(s) .
Oysters live attached to substrate in estuaries and can experience low oxygen and elevated carbon dioxide conditions,especially in midsummer.Oysters also suffer high mortality in the summer.Researchers wanted to find out if the low-oxygen or elevated carbon dioxide conditions compromised the immune system of oysters.The immune system of oysters includes cells called hemocytes that phagocytose foreign particles and remove them from tissues.These phagocytic hemocytes also initiate the production and release of reactive oxygen intermediates (ROIs) ,including HOCl,O₂⁻,and H₂O₂.ROIs attack and kill invading cells.The researchers maintained oysters at normal oxygen conditions and low-oxygen conditions at a level commonly found in estuaries in the summer.While varying oxygen levels,they kept carbon dioxide concentrations constant.They also examined the effect of different carbon dioxide concentrations when oxygen levels were kept constant.They found that production of ROIs under low-oxygen conditions was only 33% of ROI production in normal oxygen conditions.There was no detectable effect of carbon dioxide levels on ROI production.Other researchers have found that the amount of phagocytosis is not affected by oxygen conditions.(J.N.Boyd and L.E.Burnett.1999.Reactive oxygen intermediate production by oyster hemocytes exposed to hypoxia.Journal of Experimental Biology 202:3135-43. )
-Refer to the paragraph on the effects of low oxygen and elevated carbon dioxide on oysters.Why did the researchers keep carbon dioxide levels constant while they varied oxygen levels and then keep oxygen levels constant while they varied carbon dioxide levels?

A) It is technically easier to vary the level in the water of one gas at a time.
B) They could only measure the level of one gas in the water at a time.
C) The effects of oxygen and carbon dioxide levels could interact.
D) Only one gas at a time varies in estuaries in the summer.