(1 pak the 1. Put the verbs in brackets into the correct tense (active or passive): (15 points) Grasslands suddenly spreading across the Arctic about 10,000 years ago (1) (help) killing off the woolly mammoth and other prehistoric mammals, (2). (suggest) a study of ancient Arctic vegetation. Climate warming after the Ice Age, prehistoric hunters, and even a comet impact (3) (be/proposed) as reasons for the extinction of the mammoth, woolly rhinoceros, and other oversized "mega fauna" that once (4) (inhabit) Siberia and North America's far northern plains. The new DNA analysis of Arctic vegetation over the past 50,000 years, published in Nature by a team (5) University of Copenhagen, (6) (lead) by Eske Willerslev of the (offer) a new wrinkle on the climate-warming theory: (not / get) enough of the right food. The great beasts vanished because they (7) Some 10,000 years ago, the researchers (8) (find), the flowering, woody plants known as forbs-including sagebrush, yarrow, mums, and tansies-disappeared from Arctic (become) more dominated by grasses. That vegetation change (10) (be) "a likely key reason for the decline and extinction of many mega fauna species," Willerslev says, by email. The new theory (11) steppes, which (9) mammoths (12) (surprise) because (be/ thought) to thrive on grass. The Nature study challenges a long-held picture of a grass-covered "mammoth steppe" covering the polar regions of Europe, Asia, and North America during the last Ice Age. In the study, the team (13) (sample) permafrost cores dating back to 50,000 years ago from 17 locations in northern Russia, Canada, and Alaska. They (14) (find) that DNA signatures in the cores indicate that the flowering species, and the tiny roundworms associated with them in the soil, once (15) (predominate) over grasses on the ancient steppes.​