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答案：B
解析：
The triangle in the figure is a right triangle with the right angle at the lower left.
Because it is a right triangle, its base and height are the two sides that form the right angle.
The area of a triangle is (1/2)*b*h.
The base of the triangle extends from point (0，0) to point (5，0). So it is 5 units long.
The height of the triangle extends from point (0，0) to point (0，3). So it is 3 units long.
Area = (1/2)*b*h
=(1/2)*3*5
=(1/2)*15
=7.5
