What can be made in a laboratory

Synthetic gem crystals have been manufactured since the late 1800s, and their production is often marked by a need for them in industrial applications outside of the jewelry industry. The first success was in producing synthetic ruby of faceting quality. Synthetic crystals are used in communications and laser technology, microelectronics, and abrasives. Because synthetics for jewelry applications can be “made to order” [i.e. consistent color and crystal shape] given the right ingredients, time, and the facilities to grow them, they are likely to be much less rare than natural gems of equal size, clarity, and saturation of color. Because of this, and because it is possible to confuse them with gems that are naturally occurring, there are strict guidelines regarding how they are marketed and sold. so that is you answer 

The correct answer would be b) volume versus temperature, because they are also directly proportional.