Questions are based on the following passage． In fact， even without humans， the Earth's climate changes．Some climate change is（36）But，as greenhouse gases are added to the atmosphere， human influence 'emerges' from natural variability．Droughts， one of the most Intensely studied climate events， are a perfect example of an（37）with both natural and human influences．Separating the（38）strengths of the influences is a challengefor scientists．However， with the large social and economic costs of droughts， it is a challenge thescientists must（39）. In a very recent study published in the Journal of Climate， authors Richard Seager and MartinHoerling cleverly used climate models forced by sea surface temperatures to（40）how much of thepast century's North American droughts have been caused by ocean temperatures， natural variability and human influences． Droughts can be caused by a （n）（41）of separate or interactional phenomena．At its root，drought results from the low（42）of water falling and sometimes higher temperatures （which increaseevaporation rates）．The beginning of drought can often be linked to variations in ocean temperatures． It is also found that the oceans can affect the atmosphere to create conditions that are（43）responsible for drought．What's more， temperature increases（44）withhuman-driven global warmingalso play a role．This（45）agrees with other researchers who have shown that， while human-emittedgreenhouse gas warming may not cause a particular drought， it can make drought come on earlier，faster， and harder than it otherwise would．