De novo phasing is a technique that structural biologists at SLAC [slack] use to gather information about a new protein's 3D shape. This technique involves the replacement of a single atom inside the protein with a traditionally non-proteinogenic [pro-teen-oh-JEN-ik] atom with similar chemical behavior. In one example, the researchers replaced a sulfur atom with what?

W) Nitrogen

X) Selenium

Y) Phosphorus

Z) Arsenic
X) SELENIUM