The right answer is C. I think, a. and d. are obviously not serious. B is not true, because Ag and Au are in the same group in the periodic table and therefore have the same number of electrons in the last level. So, they form ions with the same charge. C is true beacause Ag and Au have different numbers in the periodic table and different amount of protons and neutrons in their nucleuses.
Answer: The size.
Explanation: On the one hand the Emperor Penguin is bigger reaching up to 48 Inches (120cm) of height and 100 Lb in weight, on the other hand the Galápagos Penguin is much smaller. For example a large adult usually goes up to 21 Inches (53cm) and 5 Lb of weight.
This is directly related to the temperature of the environment in which each one develops, because a body exchanges heat in two dimensions through the surface. So, it's easier for a big creature to maintain core temperature stable.
This is really important in environments like the Arctic where temperature goes below zero and the opposite happen in the Equator where there is a tropical weather all the time.
Blend the banana peels with coffe grounds and egg shells to make a homemade fertilizer!