自工资需求弹性

这个题目怎么做的？我觉得可能没有弄清楚自工资需求弹性的计算，哪位高人知道啊，恳请告知。小人论坛币有限，聊表心意。

当均衡工资从每小时,9.00 增加到每小时,11.00时，炼钢工人的需求量从 30，000下降到 20，000，那么，这些工人的自工资需求弹性是___ ( A )

A) -2.0   B) -0.5   C)-0.4    D) -0.2

它等于需求量变化除以
价格变动 i.e. (dQ ) / (dP)

dQ = (20000-30000) / ((20000 + 30000) / 2) = -0.4
dP = (11-9) / ((11+9)/2) = 0.2

dQ / dP = -0.4 / 0.2 = 2 = A

按照这里
https://opentextbc.ca/principlesofeconomics/chapter/5-1-price-elasticity-of-demand-and-price-elasticity-of-supply/#:~:text=The%20price%20elasticity%20of%20demand%20is%20calculated%20as%20the%20percentage,the%20percentage%20change%20in%20price.&text=Therefore%2C%20the%20elasticity%20of%20demand,is%20inelastic%20in%20this%20interval.