求助，这题有点疑问，D答案呢？

nnvvp

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In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?
A. during high traffic periods
B. after broken links are re-established
C. when upper-layer protocols require high reliability
D. in an improperly implemented redundant topology
E. when a dual ring topology is in use

fulltimes

D是存在不合适的冗余，可能产生环路，这里是单播帧，可能被无限传送，多个相同的单播出现不太可能