单项选择题
A strong，often violent，northerly wind occurring on the Pacific coast of Mexico，particularly during the colder months，is called（）.
A.Tehuan tepecer B.Papagayo C.Norther D.Pampero
单项选择题 A stretch where the channel changes from one side of the river to the other is called a（）.
单项选择题 A snag or other underwater obstruction may form a（）.
单项选择题 A section of the river that is narrower than usual and is often navigable from bank to bank is a（）.
