------I’d like to book a round- trip tic
题型：选择题
问题：
------I’d like to book a round- trip ticket.
------Sorry, sir, but there are no tickets______ at the moment.
A．available
B．usable
C．suitable
D．comfortable
题型：选择题
------I’d like to book a round- trip ticket.
------Sorry, sir, but there are no tickets______ at the moment.
A．available
B．usable
C．suitable
D．comfortable
进行中序遍历的结果是________。