题目

Hank Viscardi was born without legs. He had not legs but stumps(残肢)that could he fitted with a kind of special boots. People stared at him with cruel interest. Children laughed at him and called him ‘Ape Man’ (猿人) because his arms practically dragged on the ground. Hank went to school like other boys. His grades were good and he needed only eight years to finish his schooling instead of the usual twelve. After graduating from school, he worked his way through college. He swept floors, waited on table, or worked in one of the college offices. During all this busy life, he h为了在显微镜下清楚地观察洋葱鳞片叶表皮细胞的细胞核，下列最有效的方法是（ ）A．用较大的光圈B．换成高倍物镜C．用稀碘液染色D．用凹面镜