目前，澳大利亚经济发展中的主要经济支柱是[ ]

A．农牧业    

B．工矿业    

C．服务业    

D．旅游业

Tom usually goes to school _____ foot, but Jim goes to school _____ bus. [ ]

A. on; by

B. in; on

C. by; in

D. on; on