Explanation: Sound waves are mechanical waves, meaning they require a medium (like air, water, or solids) to travel. In empty space (a vacuum), there are no particles for the sound waves to vibrate and transmit energy, so sound cannot travel. Therefore, the speed of sound in empty space is 0. Choice B, C, and D are incorrect because sound, unlike light, cannot propagate through a vacuum.

Explanation: Physical weathering (also known as mechanical weathering) is the process that breaks rocks apart without changing their chemical composition. Common mechanisms include frost wedging, abrasion, and exfoliation. Choice A, the cracking of a rock caused by the freezing and thawing of water (frost wedging), is a classic example of physical weathering. Water seeps into cracks, freezes, expands, and exerts pressure that widens the cracks, eventually breaking the rock. Choice B, sediments being transported in a stream, describes erosion and transportation, which are processes that move weathered material, but not the weathering itself. Choice C, a sandbar forming in a stream, describes deposition, where transported sediments are laid down, which is not a weathering process. Choice D, acid rain dissolving a statue, is an example of chemical weathering, where the chemical composition of the rock is changed (e.g., dissolution of calcite in marble statues by acidic water).

Explanation: Freshwater is naturally occurring water that has low concentrations of dissolved salts and other total dissolved solids. It typically includes water found in ice caps, glaciers, icebergs, ponds, lakes, rivers, streams, and underground as groundwater. Choice A, rivers, are flowing bodies of freshwater. Choice B, aquifers, are underground layers of water-bearing permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted. This is a major source of freshwater. Choice C, oceans, contain saltwater, not freshwater. Choice D, glaciers, are large masses of ice formed from compacted snow, representing the largest reservoir of freshwater on Earth.

Explanation: An Earth year is defined by the time it takes for Earth to complete one full orbit, or revolution, around the Sun. This period is approximately 365.25 days. Choice B, one Earth day, is the time it takes for the Earth to complete one full rotation on its axis (approximately 24 hours). Choice C, one light year, is a unit of distance in astronomy, representing the distance light travels in one Julian year. It is not a unit of time or related to Earth's revolution. Choice D, one lunar month, refers to the time it takes for the Moon to complete one full cycle of phases as observed from Earth, which is approximately 29.5 days. This is related to the Moon's orbit around Earth, not Earth's orbit around the Sun.

Explanation: In the scientific method, a hypothesis is a testable explanation for a natural phenomenon. It is an educated guess or a proposed solution to a problem, typically based on prior observations or knowledge, that can be subjected to empirical testing. Choice A, an observation, is a piece of information gathered through the senses or by scientific instruments; it's what you see, hear, taste, touch, or smell, not an explanation. Choice B, a variable, is a factor, trait, or condition that can exist in differing amounts or types and is measured or controlled in an experiment. Choice C, an experiment, is a systematic procedure carried out to support, refute, or validate a hypothesis.