Some graduate students in science, technology, engineering and math—or STEM—only do research, under the guidance of a mentor.
有些科学、技术、工程和数学（简称STEM）的研究生仅在导师的指导下做一些研究。
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By looking at the hair follicles under microscopes, they saw when the stem cells turned into other cell types and linked the change to the graying hair.
"Most businesses appear to have Conficker under control," he said. "They've applied patches and updated their anti-virus software to stem the spread of the worm."