Which of the following causes ozone depletion? MCQ with Answer and Explanation

Explanation:
Cathode rays are streams of fast-moving electrons, not electromagnetic waves, originating from the cathode in a discharge tube. Because they are material particles traveling in straight lines, they can be stopped by opaque objects, thus casting sharp shadows. Therefore, statements 1 and 3 are correct.

Explanation:
Biological Oxygen Demand (BOD) measures the amount of dissolved oxygen needed by aerobic organisms to break down organic material in water. Clean, unpolluted water has a low BOD, typically less than 5 ppm. Highly polluted water can have a BOD of 17 ppm or much higher, indicating severe organic contamination.

Explanation:
Graphite conducts electricity due to delocalized electrons in its structure and is chemically inert, allowing its use in electrodes for electrolysis and batteries. Diamond is an insulator. Hardness is not the reason; graphite is soft. It is opaque and black. Graphite electrodes are used in arc furnaces.