17. Which highly reactive gas was probably absent from the Earth's primitive atmosphere? (A) methane (B) carbon dioxide (C) oxygen (D) water vapor

5. The cholesterol associated with animal cell membranes (A) is attached to membrane proteins and extends into the watery environment surrounding the cell. (B) helps to keep phospholipids from being too close to one another. (C) is an abnormality resulting from a diet high in cholesterol. (D) helps solidify the membranes when the room temperature is below freezing.

17. Which highly reactive gas was probably absent from the Earth's primitive atmosphere?
(A) methane
(B) carbon dioxide
(C) oxygen
(D) water vapor

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