Children younger than 9 years should not be given tetracyclines because:

A. It may interfere with cartilage development: Tetracyclines can deposit in developing bones and teeth, potentially causing discoloration and affecting bone growth in children. B. Children become dehydrated easily: While dehydration is a concern in any illness, it's not the specific reason to avoid tetracyclines in children. C. Photosensitivity may occur: Photosensitivity can be a side effect of some medications, but it's not the main reason to avoid tetracyclines in children. D. Children's teeth may become discoloured: Tetracycline can stain developing teeth, causing permanent discoloration. This is the major reason to avoid them in young children.