阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。

    It is not a secret ____ trees are vital to our life. They provide us with food, wood and most ____ (important), oxygen. Now there is one more thing we can add to this list—blocking out harmful bacteria from water.

    The discovery ____ (make) by a team ____ (consist) of scientists from the Massachusetts Institute of Technology (MIT) and high school students ____ were seeking a natural water filter(过滤)—one that would help communities in developing countries that do not have ____ (accessible) to modern water filter systems.

    The ____ (research), led by Professor Rohit Karnik, decided to turn____ trees for help, because they could allow liquid to flow through, while blocking out air bubbles.

    They began by cutting 1.5-inch-wide sections of tree bark from the branches of a white pine tree. The people related then tested the wood's filtering ability by pouring water containing red dye particles of different sizes through. To their ____ (amaze), they found that it was effective in trapping all the particles. Greatly ____ (encourage), the team conducted another experiment, this time with water that contained bacteria. Sure enough, the sapwood held back 99% of the bacteria, allowing only 1% to flow through.