In early experiments in the mid-1900's, the source and fate of oxygen involved in photosynthesis was mostly determined through experiments that used:

W) animals in closed containers that would die unless a plant was introduced into the container

X) a heavy isotope of oxygen as a tracer

Y) fluorescent labeling of various enzymes and organic products

Z) simple accounting of the atomic mass of all the reactants and products
X) A HEAVY ISOTOPE OF OXYGEN AS A TRACER