Question

why do astronomers use astronomical units (au) instead of kilometers (km)? (1 point)
○ there are no km in outer space, only au.
○ au is used in astronomy, while km is used in physics.
○ au is used for spacing, while km is used for length.
○ large distances are easier to express in au than in km.

Explanation:

• The first option is incorrect as kilometers are a valid unit of length everywhere, including for describing space distances conceptually (even if not practical for large ones).
• The second option is wrong because kilometers are used in astronomy too (e.g., for smaller celestial distances), and AU is just a more convenient unit for large astronomical distances, not a strict division between astronomy and physics units.
• The third option is incorrect as both AU and km are units of length (or distance), and "spacing" is a type of length/distance.
• The fourth option is correct: Astronomical distances (like between planets or the Sun and planets) are extremely large. For example, the distance from the Sun to Earth is about 150 million kilometers, which is 1 AU. Using AU simplifies expressing these large distances (e.g., Jupiter is about 5.2 AU from the Sun, which is easier than writing ~780 million km).

Answer:

D. Large distances are easier to express in AU than in km. (Note: Assuming the options are labeled A - D in order, with the last option being D. If the original labeling was different, adjust the identifier, but the text is "Large distances are easier to express in AU than in km.")