This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
A 50 year old male patient complains of reduced mouth opening. Patient gives h/o RTA 1 week back. A panoramic X-ray was taken using an intensifying screen containing gadolinium and lanthanum. On interaction with X-rays, the contents of this intensifying screen emits: 
Select and explain. 
A. Red light, B. Green light, C. Blue light, D. Yellow light
B: Green light
Answer: B: Green light 
Explanation: Contemporary intensifying screens used in extraoral radiography use the rare earth elements gadolinium and lanthanum. These rare earth phosphors emit green light on interaction with X-rays.  Compared with the older calcium tungstate screens, rare earth screens decrease patient exposure by 55% in panoramic and cephalometric radiography.
White and Pharoah, Oral radiology, ed 7th, pg-34