例:ɡ—(u)—(ā)→ɡuā niú→(n)—(iú)
h—u—____→huà q—ü—ān→____
xún→x—____ sh—____—____→shuā
yǔ→y—____ kuān→k—____—____
yuán→y—____—____ l—ún→____
For years,scientists have been worried about the ________ of air pollution on the earth's natural conditions. ( )