单选题：Questionsare based on the following passage． In fact, even w

Questionsare 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 a(n)（37）with both natural and human influences．Separating the（38）strengths of the influences is a challenge for scientists．However, with the large social and economic costs of droughts, it is a challenge the scientists must（39）In a very recent study published in the Journal of Climate, authors Richard Seager and Martin Hoerling cleverly used climate models forced by sea surface temperatures to（40）how much of the past 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 failing and sometimes higher temperatures (which increase evaporation 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）with human-driven global warming also play a role．This（45）agrees with other researchers who have shown that, while human-emitted greenhouse gas warming may not cause a particular drought, it can make drought come on earlier, faster, and harder than it otherwise would．
A．associated
B．attached
C．conclusion
D．conduct
E．distinguish
F．effect
G．natural
H．partly
I．quality
J．quantity
K．relative
L．ridiculous
M．simply
N．undertake
O．variety
第（36）题选