— glass is, for all practical purposes, a solid, its molecular structure is that of a liquid.

✅ কেন সঠিক: The two parts contrast: glass behaves like a solid, but its structure is like a liquid. ‘Although’ introduces such a contrast.

❌ অন্য অপশন: ‘Because’ and ‘since’ show cause; ‘if’ shows a condition. None fits a contrast.

💡 মনে রাখুন: Contrast connectors: although, though, even though, while, whereas.