다음 글에서 밑줄 친 부분 중 어법상 어색한 것은?

blindness is one of the most difficult of human handicaps because our eyes are so important to our learning.
(a)one thing makes education possible and life more pleasant for the blind is the development of a system
(b)by which they are able to learn to read. since the blind cannot use their eyes, (c)they are taught to use the sense of touch in their fingers. (d)reading raised printed matter of the common type is very, very difficult. the special system now (e)used was invented by charles, and perfected by louis braille.

이 문제 좀 풀어주세요. 그리고 풀이 자세하게 해주세요...
저는 다 맞는 것 같기도 하고 다 틀린 것 같기도 한데.....
특히 (b)by which 와 (e)used 는 뭔가 이상한 것 같은데.. 맞는 문법인가요 ?
자세한 답변 부탁드립니다.
(a)~(e)까지이게 왜 맞고 이게 왜 틀렸는지답변 달아주시면 감사하겠습니다.