Passenger pigeons ( 旅鸽 ) once flew over much of the United States in unbelievable numbers...described flocks ( 群 ) so large that they darkened the sky for hours. It was calculated that when its population reached its highest point, there were more than 3 billion passenger pigeons...making it perhaps the most abundant bird in the world... was seen near Cincinnati. Sadly, the abundance of passenger pigeons may have been their undoing . Where the birds were most abundant, people believed there was an ever lasting supply and killed them by the thousands. The underlined word “undoing” probably refers to the pigeons'______ . A. escape B. ruin C. liberation D. evolution