17.Since man is able to tap such resources as oil and metals, why can't he develop and harness other energy resources?
既然人类能利用石油和金属这样的能源,为什么不能开发和利用其他能源呢?
18.After analysis and determination on harmonic wave from rectification units operation, and understood hazardness of harmonic wave, the design project of harness was put forward.