American structuralism started in______.

A．the early 20th century

B．the late 20th century

C．the early 19th century

D．the late 1930s and 40s

某案现场水杯上留有相对应分布的几枚血指印，一侧为一面积较大的指印，另一侧为呈纵向分布的连续三枚指印，能看出指尖朝向，如果以指尖朝前来定位，则该三枚指印是在右侧，请分析该水杯的血手印是左手所留还是右手所留？分别为哪几个指头？