TASK DEFINITION: Given a scientific passage and an answer, generate a question for the given answer.
PROBLEM: Passage: A vapor light also produces visible light by electroluminescence The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like the streetlight pictured in the Figure below , produce yellowish light. Mercury vapor lights produce bluish light. In addition to lighting city streets, vapor lights are used to light highways and stadiums. The bulbs are very bright and long lasting so they are a good choice for these places. Answer: gas

SOLUTION: When electricity is passed through solid sodium it changes into what and gives off light?

PROBLEM: Passage: The most noticeable magnetic activity of the Sun is the appearance of sunspots. Sunspots are cooler, darker areas on the Sun's surface ( Figure below ). Sunspots occur in an 11-year cycle. The number of sunspots begins at a minimum. The number gradually increases to the maximum. Then the number returns to a minimum again. Answer: 11

SOLUTION: What is the year cycle that sunspots occur?

PROBLEM: Passage: A uniform electric field makes it easier to measure the difference in electric potential energy. This energy, also called the electric potential difference is commonly referred to as the voltage , based on the unit, volt (V). To measure the voltage across some distance, it is necessary to pick a position to be the relative zero, because voltage is the change in potential difference. Any point in a system can be given the value of zero volts, but it is typically the point of a point charge or one plate in a uniform electric field as shown above. The voltage is measured using a voltmeter, which measures the electric potential difference across two points. Answer: potential difference

SOLUTION:
The voltage is measured using a voltmeter, which measures which difference across two points?