Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.
They take over the photosynthetic process for the plant.
Mutations are like genetic mutations. Say a bird species may usually have a orange beak but one bird was born with blue, that’s a mutation.
Adaptations are things you adapt to like plant species have adapted over the year to receive less and less water to live.
Natural selection is basically ‘killing off’ the weaker ones. Say a bird species has broad wings but mutations happened over time leading them to have small wings that do not allow flight, they may eventually all die off due to that. Extinction is if al those birds were to die off from that mutation.
One has cancer and the other doesnt
One type of atom
a pure substance