A Scientist Measuring the Rate of Uptake of Radiolabeled Alanine

A scientist measuring the rate of uptake of radiolabeled alanine by cultured cells finds that rates are faster when the experiments are carried out in a buffered, balanced salts solution than when the experiments are carried out in a complete (containing all nutrients required for survival and growth) medium. The most likely explanation for this observation is

A) the concentration gradient for alanine is steeper in the former case than the latter.
B) the crenation (shrinkage of cells) in the salts solution increases the surface area-to-volume ratio for the cells increasing the rate of diffusion.
C) the absence of amino acids in the balanced salts solution decreases the abundance of molecules competing for transport, and therefore increases the apparent transport rate.
D) the temperature of the balanced salt solution is higher, so rates of transport are greater.
E) no response provides a valid answer.