The figure shows three thermodynamic systems labelled (a), (b) and (c). System (a) is an open beaker, (b) is a sealed conducting vessel, and (c) is a sealed thermally insulated flask. Which system permits exchange of **energy but not matter** with its surroundings?   
Aonly
Bonly
Conly
DBoth (b) and (c)
Answer & Solution
Correct answer: B. only
By definition, a **closed** system exchanges energy but not matter — that is system (b), the sealed conducting vessel: the seal blocks matter, but the conducting walls allow heat transfer. System (a) is **open** (both matter and energy cross the boundary). System (c) is **isolated** (neither crosses). So only (b) qualifies.
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