but,and后主语省略的问题

leadership

traveling the back roads of Hungary, in 1905 Bela Bartok and Zooltan Kodaly began their pioneering work in ethnomusicology, and they were armed only with an Edison phonogrph and insatiable curiosity. 这题lzm的解释中说they应该省去，应该and前后主语是相同的 At ground level, ozone is a harmful pollutant, but in the stratosphere it shields the Earth from the most biologically harmful radiation emitted by the Sun, radiation in the ultraviolet band of the spectrum. 这句中but前后主语也相同，it为什么不能省？

lololo

这里it前面有方位状语 如果省略 shields 有做stratosphere谓语的歧异 而且结构还不完整 如果改成 Ozone is a harmful pollutant at ground level but shields the Earth from the most biologically harmful radiation emitted by the Sun, radiation in the ultraviolet band of the spectrum, in the stratosphere. 是不是就可以解释你的疑虑了？