C
分析:如图所示,设平行四边形的底和高分别为a和h,又因A、B都是中点,则S△ADE=S△EFB=
![](http://thumb.1010pic.com/pic5/latex/96.png)
ah,又因AC=
![](http://thumb.1010pic.com/pic5/latex/13.png)
a,其对应高为
![](http://thumb.1010pic.com/pic5/latex/13.png)
h,所以S△ABC=
![](http://thumb.1010pic.com/pic5/latex/112.png)
ah,再据阴影部分的面积=S平行四边形EFCD-S△ADE-S△EFB-S△ABC,据此即可求解.
![](http://thumb.1010pic.com/pic5/upload/201305/51d62a34a9359.png)
解答:设平行四边形的底和高分别为a和h,
又因A、B都是中点,则S△ADE=S△EFB=
![](http://thumb.1010pic.com/pic5/latex/96.png)
ah,
又因AC=
![](http://thumb.1010pic.com/pic5/latex/13.png)
a,其对应高为
![](http://thumb.1010pic.com/pic5/latex/13.png)
h,
所以S△ABC=
![](http://thumb.1010pic.com/pic5/latex/112.png)
ah,
阴影部分的面积为:ah-(
![](http://thumb.1010pic.com/pic5/latex/96.png)
ah×2+
![](http://thumb.1010pic.com/pic5/latex/112.png)
ah),
=ab-
![](http://thumb.1010pic.com/pic5/latex/5192.png)
ah,
=
![](http://thumb.1010pic.com/pic5/latex/597.png)
ah,
![](http://thumb.1010pic.com/pic5/latex/597.png)
ah÷ah=
![](http://thumb.1010pic.com/pic5/latex/597.png)
;
答:阴影部分的面积是平行四边形面积的
![](http://thumb.1010pic.com/pic5/latex/597.png)
.
故选:C.
点评:此题主要考查三角形和平行四边形面积的计算方法,利用“阴影部分的面积=平行四边形的面积-3个空白三角形的面积”即可求解.