The Salk vaccine is a major factor in the fight to eradicate polio.

Formerly , in the United States, many nurses worked as private duty nurses rather than in hospitals.

Eyespots, the most rudimentary eyes, are found in protozoan原生动物 flagellates 鞭毛虫 , flatworms 扁形虫 , and segmented worms片段 蠕虫 .

Despite the proliferation of other faster modes of transportation, the railroads remain the largest carriers of intercity freight in the United States.

Insectpestsareamongtheleadingcausesofcropfailure.