CSC  the best option. Mechanical do only from IIT or old nits and bits

Depends upon your intrest in which field u really enjoy

Depends upon your interest you should decide it what may suit you. Computer science has slightly higher cutoff as compared to mechanical. The major question is about job. Computer science has good placements, but don't have job security. Mechanical is  evergreen field but now a days there are many mechanical engineers passing out every year. So probability of getting job is moderate.
Hope it may help you.