✦正面思考：
第一步：回答PES（price elasticity of supply）定义、elastic与inelastic的区别。
Price elasticity of supply:a measure of the responsiveness of quantity supplied to a change in price. When PES>1, it is elastic; when 0<PES<1, it is inelastic.【k2】
第二步：结合材料去判断black tea的供给价格弹性是elastic还是inelastic。
Price elasticity of supply of black tea is likely to be inelastic.【An1】This is because：
❶The world’s main producer--India and Sri Lanka have no extra land available for tea cultivation and Kenya has very little land for planting tea. The land for tea gardens is very difficult to find. Some land is available in Rwanda and Vietnam but this takes time to develop.【Ap1】
❷Tea bushes can only be harvested six years after they are planted. So the supply can’t keep the pace of the demand. 【Ap1】
❸The stock can reduce price fluctuation. But buffer stock scheme was never attempted because of major obstacle of funding, storability and the difficulty of achieving agreement from tea growers.【Ap1】
✦反面思考：
第三步：逆向思维，black tea的供给有可能会是inelastic的（这里是结合题目中的暗示），或者是uncertain（这里是需要自己结合行业背景进行合理发挥）。
In the long run, Rwanda and Vietnam may develop more land to produce black tea. And technical progress may solve the difficulty of stock or reduce the period of planting. In these case, the PES may become elastic. 【Ev2】
Besides, tea plantation depends on climate. If there is drought or flood, the supply will decrease. And farmers’entry and exit to the tea market also influences PES. So there is no guarantee that PES will be elastic in the long run. 【Ev2】
（做笔记：长短期思考方式所带来的不同结论正是常见的evaluation得分点。）
