Answer: D) nutrients and other life-sustaining molecules are in limited supply and must be continually recycled Explanation: Biogeochemical cycles are crucial to ecosystem function because ::   1. Chemical elements are available in only limited amounts and must be recycled. 2. Nutrients and other life-sustaining molecules are in limited supply and must be continually recycled.

Answer: A) a nonpolar molecule with polar bonds Explanation: The four bonds of carbon tetrachloride (CCl4) are polar, but the molecule is nonpolar because the bond polarity is cancelled by the symmetric tetrahedral shape. Hence, making the net dipole as zero and molecule as non-polar.   When other atoms substitute for some of the Cl atoms, the symmetry is broken and the molecule becomes polar.