一棵深度为k的平衡二叉树,其每个非叶子结点的平衡因子均为0,则该树的结点数是______。
A. 2k-1-1
B. 2k-1
C. 2k-1+1
D. 2k-1
At last she left her house and got to the airport, only ______the plane flying away.
A. having seen
B. to have seen
C. to see
D. saw
Rosamond felt unhappy without any ______ of his project at the meeting.
A. clue
B. hint
C. comment
D. mention
We work to make money, but it's ______ that people who work hard and long often do not make the most money.
A. dilemma
B. conflict
C. prejudice
D. paradox