Which structures in the lymph vessels ensure that lymph flows in one direction? valves lymph nodes ducts lymphocytes

Option A,  Valves

Explanation:

Lymphatic vessels are valved structures as they carry thin walled valves in it. Valves allow one way movement of lymph fluid. As the pressure inside a lymphangion increases, the semilunar valve opens and allows the movement of lymph fluid  into another lymphangion. This semilunar valve prevents the backward flow of the lymph fluid and thus a lymphatic system requires no central pump to operate efficiently.

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the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.