Correct Answer: A

Rationale: To find the perimeter of a rectangle, you add up all its sides. In this case, the length is 3 units greater than the width, so the length can be expressed as W + 3. The formula for the perimeter of a rectangle is 2W + 2(L), where L represents the length. Substituting W + 3 for L, the correct expression for the perimeter becomes 2W + 2(W + 3), which simplifies to 2W + 2W + 6 or 4W + 6. Choices B, C, and D do not correctly represent the formula for the perimeter of a rectangle. Choice B simply adds the width twice to 3, neglecting the length. Choice C multiplies the width by the sum of the width and 3, which is incorrect. Choice D combines the width and 3 times the width, which is not the correct formula for the perimeter of a rectangle.

Correct Answer: A

Rationale: The correct expression for the sum of three times a number and eight times a different number is given by 3x + 8y. This represents adding three times the variable x (3x) to eight times the variable y (8y). Choice B (8x + 3x) is incorrect as it represents adding eight times x to three times x, which is redundant. Choice C (3x - 8y) is incorrect because it represents subtracting eight times y from three times x, not their sum. Choice D (8x - 3y) is also incorrect as it represents subtracting three times y from eight times x, not their sum.

Correct Answer: B

Rationale: The correct answer is B: 1. The average width of a doorway typically ranges from 0.8 to 1.2 meters, making 1 meter a reasonable estimate. Choice A (0.5) is too narrow for a standard doorway. Choice C (10) is too wide for a typical doorway. Choice D (3) is also wider than the standard width of a doorway.

Correct Answer: B

Rationale: The independent variable in this scenario is 'Stocks.' An independent variable is the one that is manipulated or controlled by the experimenter. In this case, stocks are the factor that is changing and influencing the other variables - production, sales, and investments. Production, sales, and investments are dependent on the changes in stocks; hence, they are the dependent variables. While production, sales, and investments may increase as a result of changes in stocks, the stocks themselves are the driving force behind these changes, making them the independent variable.

Correct Answer: A

Rationale: The correct answer is A: 1000 mL. This is a standard conversion in the metric system where 1 liter is equivalent to 1000 milliliters. Choice B, 100 mL, is incorrect as it represents only a tenth of a liter. Choice C, 10 mL, is incorrect as it represents only a hundredth of a liter. Choice D, 1 mL, is significantly less than a liter, as it is only a thousandth of a liter.