除去某溶液里溶解的少量杂质，下列做法中不正确的是（括号内的物质为杂质）[ ]

A．NaCl溶液(Na₂CO₃)：加适量盐酸后加热

B．KNO₃溶液(AgNO₃)：加KCl溶液，过滤

C．NaCl溶液(BaCl₂)：加过量Na₂CO₃溶液，过滤再加适量盐酸并加热

D．KNO₃溶液(NaCl)：加热蒸发得浓溶液后，降温

In relation to contract law explain the meaning and effect of:

(a) an offer; (4 marks)