Europeans are being dug out from heavy snow after a week of very cold weather. The snowstorm is the harshest to hit Europe. Airports were shut down. And schools have been closed for a week.
    Until Wednesday, at least 400 people across Europe had died because of the cold temperature. Several countries have said a state of emergency. In Ukraine, temperatures dropped as low as -33℉.
    Although the situation is bad, at least one country is hoping the weather will last a little longer. In the Netherlands, the government is preparing for the country’s “11 Cities Tour”. The 125-mile ice-skating marathon will be held along a network which joins 11 towns and cities together in Friesland Province.
    The ice along the way must be inches thick for the event. Up to 2 million people are thought to cheer on the race's 16,000 competitors.
On Wednesday, Dutch soldiers joined in the preparations. They cleaned snow along the way. This will be the 16^th time that the event has taken place since the first race in 1909. Dutch General Hans said his soldiers were happy to lend a hand. “We want to do our bit for this national party.” he said.

加拉帕格斯群岛的地雀原来属于同一个物种，从南美大陆迁来后，分布在不同的海岛上，从而形成了____ ．这样，不同的种群就可能出现不同的____ ，且互不影响，从而使不同的种群的基因频率发生不同的变化．同时由于各岛屿上的条件不同，所以自然选择对各种群所起的作用也不相同，久而久之，各种群的____ 就会形成明显的差异，并逐步出现____ ，从而使原来属于同一个物种的地雀，就成了不同的物种．由此可见，隔离是物种形成的必要条件．上述一个物种进化成多个不同的物种，体现了生物多样性中的物种多样性．生物多样性还包括基因多样性和____ 多样性．

已知方程组 的解满足x＋y=3，则k的值为（　　）