【正确答案】：
（A）THE AMBIGUITY HERE IS CAUSED BY POLYSEMY. SENTENCE（A）IS QUITE AMBIGUOUS BECAUSE WE HAVE NO WAY TO DETERMINE WHETHER JOHN 'PARTICIPATED' IN THE RACE OR 'ORGANISED' THE RACE AS THE WORD RUN CAN MEAN BOTH.
（B）THE AMBIGUITY HERE IS CAUSED BY HOMONYMY. DUCK CAN BE UNDERSTOOD AS A NOUN MEANING A “KIND OF POULTRY” OR AS A VERB MEANING “LOWER ONE’S HEAD OR BODY QUICKLY, DODGE”, THIS AMBIGUITY CAN BE ELIMINATED BY ALTERING THE CONTEXT A LITTLE AS “THEY SAW HER DUCK SWIMMING IN THE RIVER.” 
【题目解析】：（a）这里的歧义是由一词多义引起的。句子（a）是相当模糊的，因为我们无法确定约翰是“参加”了比赛还是“组织”了比赛，因为“跑”这个词可以同时表示两者。（b）这里的歧义是由同音异义词引起的。"duck"可以被理解为意为“一种家禽”的名词或意为“迅速低下头或身体，躲避”的动词，这种歧义可以通过稍微改变上下文来消除，如“他们看见她的鸭子在河里游泳”。