在1和256之间顺次插入三个数a，b，c，使1，a，b，c，256成一个等比数列，则这5个数之积为（ ）
A．218
B．219
C．220
D．221
因为1，a，b，c，256成等比数列
根据等比数列的性质得：b2=ac=1×256，a2=1×b＞0，
则b=16，ac=256
∴这5个数之积为256×256×16=28×28×24=220
故选C．
| 完形填空。 | |||
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