A sample of nitrogen in a glass bulb has a mass of 245 mg. What is this mass in kilograms? (Note: 1,000 mg = 1 g, 1,000 g = 1 kg.)

Convert milligrams to grams, then grams to kilograms: 245 mg ÷ 1,000 = 0.245 g, and 0.245 g ÷ 1,000 = 0.000245 kg. Therefore, the mass is 0.000245 kg, choice B.

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