am a btech final yr studnt. if i write d cat dis year and suppose i score good, can i get into an iim immediatly aftr my btech nxt year?

since the score of cat is only valid for one year, i want to know how the admission works. bcoz final year btch studnts can write cat, but the score is valid for a year only aswell. so then when does the admission to iim take place? vats d point if i write cat this yr, vn i'll pass out only nxt yr?