1. This is a major achievement because, in relative terms, planets are very small and do not emit light.
    这是一个重要的成就，因为相对来说，行星很小，而且也不发光。

2. It has been found that certain bats emit squeaks and by receiving the echoes,
    人们发现，某些蝙蝠能发出尖叫声，并能通过回声