—What color is your book?
—__________ red.
A:They are
B:It's
C:This is
D:That is
如图,以直角三角形a、b、c为边,向外作等边三角形,半圆,等腰直角三角形和正方形,上述四种情况的面积关系满足S1+S2=S3图形个数有( )