The aurora borealis (northern lights) are caused by
a. sunlight reflecting from polar ice
b. emissions from atoms and molecules in the upper atmosphere excited by electrons from the sun
c. thunderstorms
d. an unknown process
Question 2
Lasers are used
a. to produce holograms
b. in surgery
c. to measure distances between objects accurately
d. in DVD and compact disc players
e. all of the above