As winter approaches, temperatures begin to drop. The days grow shorter, and squirrels are seen gathering nuts and burying them. Birds fly in flocks toward the south, but some animals, like mountain hares, do not mind the cold and instead adapt by turning their fur white. Some insects, like the morning cloak butterfly, seek shelter in tree holes or other shelters. Then, when temperatures slowly climb back up, many animals return to their homes and come out of hiding. Which of the following is the main external stimulus in the passage?

A.
The changing seasons
B.
The animals leaving
C.
The animals hiding
D.
The diversity of animals

percent abundance for cu-63 can be shown as:

63 x + 65 (1-x) = 62.9296

65 - 62.9296 = 2 x

x = 1.035

thus, percent abundance for cu-63 is 1.035

percent abundance for cu-65 can be shown as:

63 x + 65 (1-x) = 64.9278

65 - 64.9278 = 2 x

x = 0.0361

thus, percent abundance for cu-63 is 0.0361

the mass spectrum of cu-63 showing the intense peak as, this isotope is found in nature more abundantly around 92.38 % in comparison to cu-65 which is found 62.58 % naturally

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.

the answers are

c and c