常见表达：*one theory is that…/ the first hypothesis is…, but….
e.g. （tpo35 L3)
…By about 10,000 B.C.E., all those giant creatures, the Megafauna of North America were gone. We don't know exactly what happened to them, but there are some theories（提出理论的不确定性）.
One theory(提出第一种可能性理论，听的过程中要准备好后面可能会出现该理论的驳斥信息) is that they were hunted to extinction by humans. The humans who coexisted with these giant species in North America at that time were what we today called the Clovis People. And there is a Clovis site in a valley in southern California where the remains of thirteen mammoths were found.…That would appear to be some pretty compelling evidences.But then(提出质疑：Clovis一般猎杀小型动物，可能不具备猎杀猛犸象等大型动物的能力) at other Clovis sites, there's also a lot of evidence that the Clovis people mostly gather plants and hunted small game, like rabbits and wild turkeys…