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.
Water only needed for Photosynthesis.. Not both of Photosynthesis and Respiration..
That would be A. Produces reproductive organs which produce a zygote.
Zygotes grow into the sporophyte(diploid), spores grow into the gametophyte(haploid).
Sporophyte produces haploid spores through meiosis, gametophyte produces gametes through mitosis.