Correct Answer: B

Rationale: Collusion refers to a secret agreement or conspiracy, often for deceitful or illegal purposes. While choices A, C, and D may involve some level of agreement or understanding, collusion specifically implies a secretive and usually dishonest arrangement, making choices A, C, and D incorrect in this context.

Correct Answer: B

Rationale: The correct answer is B. Diameter is defined as the distance across the center of an object, typically a circle. Choice A is incorrect because width refers to the measurement of something from side to side, not necessarily through the center. Choice C is incorrect because height is a measurement from top to bottom, not across the center. Choice D is incorrect because the radius of a circle is the distance from the center to any point on the circle's circumference, not the distance across the center.

Correct Answer: A

Rationale: Concave means rounded inward. When something is concave, it curves inwards like the interior of a bowl. Choice B, 'Rounded outward,' is incorrect as this describes a convex shape, not concave. Choice C, 'Straight,' is incorrect as concave refers to a curved shape, not a straight one. Choice D, 'Flat,' is also incorrect as it does not describe the inward curve characteristic of concave shapes.

Correct Answer: B

Rationale: The correct answer is B: 'Vague.' Nebulous means something that is vague or unclear. Choices A, C, and D are incorrect because nebulous is the opposite of clear, distinct, and complex. Nebulous refers to something lacking definite form or limits, making it difficult to understand or perceive clearly.

Correct Answer: B

Rationale: The correct answer is B: 'Able to be heard.' Audible refers to something that can be heard. Choice A, 'Impossible to hear,' is the opposite of the meaning of audible. Choice C, 'Visible,' is unrelated as it pertains to sight, not hearing. Choice D, 'Silent,' is also not correct as it refers to the absence of sound, which is different from something being able to be heard.