Related Clues:

• C, O, or Co
• Jr. high preceder
• ANAGRAM #2 (with 47-across)
• Tin or tungsten
• Boron or bohrium: Abbr.
• Neon or nobelium, e.g.
• See 39 Across
• I, O or U, but not A and E
• Xenon or neon
• U, V or W
• Primary school: Abbr.
• Comfort zone
• Carbon or copper
• Hydrogen or helium
• Chemical building block