Can you help me with the Official Test 4 Question 21?

Yep! I actually think it’s helpful to do a little drawing here. Below is my very rough (sorry, had to draw it on my laptop trackpad) drawing of the axes as described in the question, with year 2000 data on the x-axis and year 2010 data on the y-axis. I’ve also drawn the yx line.

Now let’s pick a set of bars and try to plot it. Looks like the bars for wood were at about 2.25 quadrillion Btu in 2000 and a little less than 2 quadrillion Btu in 2010.

Biofuels, on the other hand, were fairly small in 2000 (around 0.25) and larger in 2010 (say around 1.8).

Do you see what’s going on here? When consumption of an energy source grew from 2000 to 2010 (like biofuels), its dot will be above the yx line. When consumption declined from 2000 to 2010 (like wood), that dot will below the yx line.

Now you can stop plotting and just count the energy sources for which the 2010 bar is higher than the 2000 bar. I’m seeing three: biofuels, geothermal, and wind. So 3 is your answer.