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。

    The truth ____ trees are vital to our life is not a secret. They provide us withfood, wood and most ____(important), oxygen. Now there is one more thing we canadd 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—onethat would help communities in developing countries that do not have ____(accessible)to modern water filter systems.

    The ____(research), led by Professor RohitKarnik, decided to turn to trees for help because they could allow liquid toflow through, while blocking out air bubbles.

    They began by cutting 1.5­inch­wide sectionsof tree bark from the branches of a white pine tree. The people related thentested the wood's filtering ability by pouring water containing red dyeparticles of different sizes through. ____ their amazement, they found that itwas effective in trapping all the particles. ____(encourage), the teamconducted ____ experiment, this time with water that contained bacteria. Sureenough, the sapwood held back 99% of the bacteria, allowing only 1% to flowthrough.