Which statement most accurately describes Countercurrent multiplier?

A. A loop-of-Henle mechanism that establishes the medullary osmotic gradient for urine concentration. B. Release of a mature oocyte from the dominant follicle after the mid-cycle luteinising hormone surge. C. An increase in ventricular filling produces a stronger contraction within physiological limits. D. A renal hormone that stimulates red-cell production in bone marrow during hypoxia.