彈性相關問題 - 經濟

題目如下：
As a result of a decline in interest rates and a rise in household income,
the demand curve for housing has shifted to the right, but has retained the
same slope. Consequently, the elasticity of demand for housing
(A)has declined (B)has increased
(C)has remained unchanged (D)cannot be compared

答案是A，我問的老師跟我解釋是說因為只有外生變數變動內生變數不變，
所以P固定。在P不變的情況下需求線平行右移，彈性下降。

但我有個疑惑，當需求線右移的時候，考慮供給的狀況下，
成交的價格跟數量應該也會改變，而這題為什麼不用考慮供給面呢？

煩請回答，謝謝！<(_ _)>

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