When asked why he prefers bikes to buses, the professor said,  “Riding a bicycle takes __________ half an hour, but it’s more convenient and environmentally friendly. ”

A．absolutely

B．accurately

C．approximately

D．exactly

历史上引起我国人口迁移的直接原因是[ ]

A、移民支边、战争和自然灾害　　　

B、政府采取措施强制移民

C、生存的自然环境恶化　　 　

D、海外的就业条件好