Both C and D are true. Consider the table below. More heat is passed from the veins to the arteries going to the muscle. The gradient in heat between veins and arteries comes to equilibrium more quickly in counter-current exchange. Unidirectional flow prevents heat exchange.
Unidirectional flow is actually more efficient.
Read the question carefully and respond accordingly. The probability of extinction of subpopulations within a metapopulation depends on several factors. Name four of these factors and describe how each could increase the probability of extinction. Thus, sink habitats have a greater probability of extinction. To determine if population is growing, calculation of r is needed.