Look at this desk. It's Jim's desk. It's brown. Jim's book is on it. The book is his English book. His pencil-box is on it, too. His pencil is in his pencil-box. But where's his pen? He can't find(找到) it.

氢气是一种理想的燃料，它具有①资源丰富②产热高③燃烧产物是水，不污染环境这三大优点，从社会进步和发展要求的角度，你认为它作为一种能源和现有其它能源物质比较，最大的一条优点是　____ 　（填序号）．目前，世界各国科学家正致力于研发氢气的获得技术，我们知道电解水可产生氢气，但此法不宜大面积使用，理由是____ ，

你认为科学家们目前主要研究点在哪里？____ 

读左图“延安1951～1991年旱涝灾害发生频率”图和地表景观图，完成下列各题。