Which of the following is a fundamental unit in the SI system? MCQ with Answer and Explanation

Explanation:
Lactometer is a specialized hydrometer for measuring milk purity/density. It works on Archimedes' principle: pure milk has specific density; adulteration (with water) changes density, altering lactometer reading. While it measures density, its specific application is milk testing. Option B is partially correct but not specific; competitive exams expect the precise application. Memory tip: 'Lacto = milk; meter = measure'. This application-based question tests knowledge of scientific instruments in daily life, frequently appearing in competitive exams. Always note context-specific uses of general instruments.

Explanation:
Efficiency η = useful work output / total work input = useful power output / total power input, since power = work/time. Both definitions are equivalent for steady operation. Efficiency is always < 1 (or

Explanation:
Alpha (α) radiation: helium nucleus (²He⁴, 2p+2n), +2 charge, low penetration. Beta: electrons; Gamma: photons; Neutron: neutral particle. Memory aid: 'Alpha = He nucleus; Beta = electron; Gamma = EM wave'. Radioactivity classification frequently tested in competitive exams to distinguish radiation types.