Correct Answer: A

Rationale: To find the product of 0.5 and 20, you multiply them together. 0.5 x 20 = 10. This calculation involves multiplying a decimal (0.5) by a whole number (20). Choice A, '10', is the correct answer. Choice B, '15', is incorrect as it does not accurately represent the result of multiplying 0.5 by 20. Choices C and D, '20' and '25', are also incorrect as they do not reflect the correct calculation. This question assesses your ability to perform basic arithmetic operations involving decimals and whole numbers.

Correct Answer: B

Rationale: The correct answer is B: Liters. In the metric system, liters are the base unit for liquid volume. Milliliters, though a metric unit, are a smaller unit of volume and not the base unit. Gallons, on the other hand, are part of the imperial system and not a metric unit for volume. Option D is a duplicate of option B, making it incorrect.

Correct Answer: B

Rationale: To convert milliliters to liters, you need to divide by 1000 since there are 1000 milliliters in a liter. Therefore, 250 milliliters is equal to 250/1000 = 0.25 liters. Choice A, 25 liters, is incorrect as it is a magnitude higher than the correct answer. Choice C, 2.5 liters, is also incorrect as it is 10 times greater than the correct conversion. Choice D, 0.025 liters, is incorrect as it is a magnitude lower than the correct conversion.

Correct Answer: A

Rationale: The correct answer is A: 10^-9 meters. 1 nanometer (nm) is equivalent to 10^-9 meters. This conversion is commonly used in various scientific fields, including nanotechnology, to represent extremely small lengths. Choice B, 10^-6 meters, is incorrect as it represents a micrometer, not a nanometer. Choice C, 10^-12 meters, is incorrect as it would be a picometer, which is smaller than a nanometer. Choice D, 10^-15 meters, is incorrect as it would be a femtometer, much smaller than a nanometer.

Correct Answer: A

Rationale: To convert grams to kilograms, divide by 1,000 since there are 1,000 grams in a kilogram. Therefore, 34.5 grams · 1,000 = 0.0345 kilograms. The correct answer is 3.45 kg (not 3.45 grams, which is the amount given in the initial question). Choices B and C are incorrect as they do not involve the conversion from grams to kilograms. Choice D is also incorrect as it presents the value in grams instead of kilograms.