Whydon’tbirdsgetlostontheirlongmigratory(迁居的)flights?Scientistshavepuzzledoverthisquestionformanyyears.Nowthey’rebeginningtofillintheblanks. |
A.havebeenknowntoscientistsformanyyears |
B.areknownbyeveryone |
C.haveonlyrecentlybeendiscovered |
D.willprobablyremainamystery |