Why do we use the half-way mark as a point of comparison rather than the point at which all the disks are floating?

b. the carbon cycle ensures living things have a source of hydrogen

answer; i believe the answer is (polygenic inheritance;

because the light is a great source of energy for the plants to survive. (i hope this )

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.