有关一道改错题!高手请进!The insects are so small that they are difficul

有关一道改错题!高手请进!
The insects are so small that they are difficult to be seen without using a microscope.
我是这样改的
The insects are so small that they are difficult to see without using a microscope.
因为我记得这样的例句：This book is hard to read.
但是我不知道其中的原因和理由, 请高手指点!