Which of the following statements is true?

a) cool air is less dense than warm air.
b) sound does not require a medium for travel.
c) magnetism is caused by the spinning motion of electrons.
d) electricity flows from positive to negative areas.

Flock x flock y flock z total pieces of food eaten (from previous page) 147 93 60 food percentage* % % % simulated number of birds in flock for 3rd generation** * divide each flock's total pieces of food by 300, the total number of pieces of food eaten. ** multiply the food percentage for each flock by the total number of birds (30).

How many species of living things are alive on earth today? more than 1,000,000 none of these answers are correct 100-500 1,000 –500,000 o 500,000 -1,000,000

What were the main components of earth’s earliest atmosphere? oxygen and ammonia hydrogen and helium oxygen and nitrogen hydrogen and nitrogen