Solar energy will not run out (unless the sun burns out), but the supply of fossil fuels we use is slowly depleting. Burning fossil fuel also releases greenhouse gas into the air, which is bad for the atmosphere.
Period are going left to right across the periodic table
Groups are going up to down on the periodic table
Least dense means the one that is at the top. Wood floats on top of water, so wood would be the least dense. Water has a density of 1 and the others would all sink, so they're more dense. The answer is wood
