30.假定A为自变量,B为因变量,弹性的数学表达式为( )

A、<span style="font-size: 16.8px; line-height: 25.2px; background-color: rgb(255, 255, 255);">E=（ΔB/ΔA）·（A/B）</span>

B、<span style="font-size: 16.8px; line-height: 25.2px; background-color: rgb(255, 255, 255);">E=（ΔA/ΔB）·（B / A）&nbsp;</span>

C、<span style="font-size: 16.8px; line-height: 25.2px; background-color: rgb(255, 255, 255);">E=（ΔB/ B）·（ΔA/A）</span>

D、<span style="font-size: 16.8px; line-height: 25.2px; background-color: rgb(255, 255, 255);">E=（ B /ΔB）·（A/ΔA）</span>