cloning human embryos is not legal in many countries, some scientists are already pushing ahead with research in order to produce a cloned human baby.
A. While
B. When
C. Since
D. if only
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(答案→)A. While
B. When
C. Since
D. if only
解析：考查连词：A. While虽然,B. When当…时候，C. Since既然D. if only 但愿，句意：虽然克隆人类胚胎是违法的，但一些科学家已经在努力推进研究为了制造克隆的人类婴儿。选A。