Why arteries have thick and elastic muscular walls?

The thick, elastic walls allow the heart to withstand the pressure waves from the strong blood pressure. Veins have thinner ones because they don't receive much pressure.

the sun d the answer is sun

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.