The correct option is (a).
Explanation
Air is a mixture of gases that constitutes the atmosphere. It possesses physical properties such as mass, density, and pressure, which are utilized in various natural and mechanical processes, including aerodynamics and separation techniques.
Statement 1 is Correct: Air provides the necessary medium for flight. Birds and bats rely on air resistance and air currents to generate lift and thrust. The physical interaction between their wings and the air allows them to maneuver and stay aloft.
Statement 2 is Incorrect: Airplanes rely on the presence of air to generate lift. The shape of an airplane's wings (airfoils) causes air to move at different speeds over the top and bottom surfaces, creating a pressure difference (Bernoulli's principle) that lifts the plane. A vacuum represents the absence of air, which would render aerodynamic flight impossible.
Statement 3 is Incorrect: Air is an effective medium for separating mixtures where components have different weights, such as sand and sawdust. This method is known as winnowing. When moving air is applied to the mixture, the lighter sawdust particles are blown away, while the heavier sand particles fall vertically, resulting in effective separation.
Key Takeaway: Air is a vital abiotic component that enables aerodynamic flight through the generation of lift and facilitates separation processes like winnowing by acting on the density differences of materials.