Bird lovers know that a good place to go bird-watching is Guandu Nature Park. As many as 220 kinds of birds come to this park every year. Since the park offers birds lots of food to eat and places to build their nests, it has become a paradise（天堂）for birds.

    Guandu Nature Park is also a great outdoor classroom. The park's Nature Center provides guided tours to the public. During the tour, visitors can learn about the environment. So, if you want to have a fun and educational weekend, come to Guandu Nature Park. Get away from city life and head into nature.

某林场去年年末有森林木材量为a，木材以每年25%的增长率生长，而每年冬天要砍伐的木材量为x．从今年起，为了实现到第20年年末木材的存有量达到4a的目标，则x的最大值是多少？（取lg2=0.30）

Sue was so _______ her holiday that she couldn't stop talking about it.

由于蟹类的捕食，某种海洋蜗牛种群中具有较厚外壳的个体的比例逐渐增加。对这个现象的解释，正确的是(     )