题目内容：
根据以下资料，回答题。
First, the spotted owl was threatened by logging in the Pacific Northwest．Now it's in danger from a new enemy, the barred owl．Barred owls have been moving to the Northwest from the eastern part of the United States．Stan Sovern has studied spotted owls．Now when he calls for spotted owls, barred owls are starting to appear．Sovern threw a mouse on the ground, and a barred owl grabbed it.Scientists have learned that spotted owls start to vanish when barred owls come．Some barred and spotted owls have mated and produced hybrid babies．One spotted owl was killed by a barred owl.
Professor Ned K．Brown of the University of California-Berkeley says, "In some areas of Washington, the barred owls moved into very dense, deep woods．The same kind of woods that arc opened up, or destroyed by logging, that adversely influences the spotted owls."
Ten years have passed since the federal government began protecting the spotted owl．Loggers were forced to limit logging on seven million acres of government land．No one is sure how the arrival of the barred owls will impact laws that protect spotted laws．The barred owls like to live in the deep forests where loggers like to cut down trees．The barred owls will likely keep migrating to the Northwest. Scientists learned about the barred owl migration by __. A． decoding spotted owl sounds
B． feeding mice to the owls
C． monitoring spotted owl populations
D． researching the Internet
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