The distance between planets varies as they move in their orbits. The minimum distance from the Earth to Mars is about 0.35 AU. About how many kilometers is that?

One astronomical unit (AU) is about 150,000,000 km. Multiply by 0.35: 0.35 × 150,000,000 = 52,500,000 km, which is closest to 55,000,000 km. So the correct choice is 55,000,000 km.

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