单选题 Why it is difficult to say exactly how many miles of improved waterways are in operation?

A、 The most recent figure is for 1975 and the figure is declining.
B、 Water transport's market share goes up and down frequently.
C、 So few miles of inland waterways exist that no one has bothered to count them.
D、 You must know whether to include coastwise and inter-coastal shipping in the figure.

【正确答案】 D

【答案解析】根据第三段第一句“The exact miles of improved water in operation depend in part on whether coastwise and inter-coastal shipping are included”，就可断定[D]正确。