Question 37 - Reading and Writing Practice Test for the PSAT/NMSQT Exam

What punctuation should be used in the underlined portion of the last sentence of the passage provided?

William Ramsay, a Scottish chemist, won the Nobel Prize in chemistry in 1904 for his work in identifying the noble gases. These inert gases were added to the periodic table, which organizes chemical elements based on their atomic number. There are six naturally occurring noble gases helium, neon, argon, krypton, xenon, and radon.