Correct Answer: C

Rationale: To find the sum of all six numbers, we multiply the average (6) by the total numbers (6), which equals 36. Since the sum of five numbers is 25, the sixth number can be found by subtracting the sum of five numbers from the total sum: 36 - 25 = 11. Therefore, the sixth number is 11. Choice A, 8, is incorrect because adding 8 to the sum of five numbers (25) would result in a total greater than the correct sum of all six numbers (36). Choice B, 10, is incorrect because adding 10 to the sum of five numbers (25) would also result in a total greater than the correct sum of all six numbers (36). Choice D, 12, is incorrect because adding 12 to the sum of five numbers (25) would exceed the correct sum of all six numbers (36).

Correct Answer: C

Rationale: To find the sum of all six numbers, we multiply the average (6) by the total numbers (6), which equals 36. Since the sum of five numbers is 25, the sixth number can be found by subtracting the sum of five numbers from the total sum: 36 - 25 = 11. Therefore, the sixth number is 11. Choice A, 8, is incorrect because adding 8 to the sum of five numbers (25) would result in a total greater than the correct sum of all six numbers (36). Choice B, 10, is incorrect because adding 10 to the sum of five numbers (25) would also result in a total greater than the correct sum of all six numbers (36). Choice D, 12, is incorrect because adding 12 to the sum of five numbers (25) would exceed the correct sum of all six numbers (36).

Correct Answer: A

Rationale: The mode of a set of numbers is the value that appears most frequently. In the given set {4, 4, 5, 7, 8}, the number 4 appears twice, which is more frequent than any other number. Therefore, the mode of this set is 4. Choice B, 5, is incorrect as it only appears once in the set. Choices C and D, 7 and 8 respectively, also appear only once each, making them less frequent than the number 4.

Correct Answer: A

Rationale: To simplify the expression (x^2 + 2x)/(x), we factor out x from the numerator to get x(x + 2) and then cancel the x in the denominator. This simplifies to x + 2, making choice A the correct answer. Choice B (x^2 + 2) is incorrect as it does not account for the division by x. Choice C (x(x + 2)) is also incorrect as it represents the factored form before cancellation. Choice D (1 + 2/x) is incorrect as it does not simplify the expression correctly.

Correct Answer: C

Rationale: A valid hypothesis must be a testable statement that predicts a relationship between variables. Option C is the only statement that presents a clear cause-and-effect relationship between the amount of poison given and the time it takes for the rat to die. Option A is descriptive without predicting an outcome, option B is a question rather than a statement, and option D is a general fact about poison and rats, lacking a specific hypothesis for testing.