（1）by:到...时候为止.到他出现的时候为止.出现这个动作发生在过去.
（2）我们已经种了10棵树.种树这个动作发生在他出现这个动作之前.
也就是说种树这个动作是过去的过去.所以用过去完成时.
答案为：B
by the time he showed up ,we （）ten trees A have planted B ha
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