A is your answer. it cant be B because H doesn't equal 2 on both sides.it cant be C because S doesn't equal 3 on both sides of the equation . It also cant be D because Au doesn't equal 3 on the first part of the equation so your best answer is A. Hope this helps!!!!
The correct answer would be through adaptation and eventually over time they had to evolve to the area around them which made them develop over time to have longer necks .