请教3道语法~ 谢了~

cindypalau

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26. Radiocarbondating of fossils taken from caves on island along southeastern Alaska’s coast suggeststhat a least a portion of the area remained ice-free during the last iceage.  (主语是啥？看不出来)





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28. Hydrocarbons,with which fruit flies perfume themselves in species-specific blends, are knownto be important in courtship, apparently assisting flies that taste thehydrocarbons on prospective mates in distinguishing their own species fromothers.

我选：apparently assisting flies to taste thehydrocarbons on prospective mates to distinguish their own species from thoseof  （为啥不可以？）





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13. As with thecase of traditional pharmacies, on-line drugstores rely on prescriptions tohave success, since it is primarily prescriptions that attract the customers,who then also buy other health-related items.

C. As is the casewith traditional pharmacies, prescriptions are the cornerstone of a successfulon-line drugstore.
（答案是C，可我选了原句。如何判断哪个正确呀?)

番番

1，主语是 Radiocarbondating of fossils 化石的放射性
2，those of 冗余。楼主横线上的是标答吗，咋么我觉得也不怎么顺畅。
3,  如果你选原句，主语就是on-line drugstores，也就是后面it的指代对象，说不过去吧，
答案c，脱了皮就是prescriptions  are....since it is....prescriptions...it指代很清晰。