Water _____ at 100 degrees centigrade.
A.boiled
B.boils
C.has boiled
D.was boiling
答案:B
Mary's pale face is a _____ that she _____ rather weak after the operation.
A.suggestion; be
B.sign; was
C.suggestion; is
D.sign; be
函数y=log2(x2+2)的值域是______.