Is it safe to reject the offer?

Soon to be CS grad. Company A -> Internship offer, need to sign soon. FTE offers pay better than B from what levelsfyi say. Company B -> FTE offer. Lower pay but still within reason. Problem is

Soon to be CS grad. Company A -> Internship offer, need to sign soon. FTE offers pay better than B from what levelsfyi say. Company B -> FTE offer. Lower pay but still within reason. Problem is Company A needs to me to sign really soon while company B just sent an email offer with some details like pay and benefits and wants me to reply if im accepting. Do I just accept both first and wait for B to finish the process and renege on A and be blacklisted on A or does an email offer means its safe to turn down others? Both clash and there is no way to postpone any start dates without risking the offer. Thanks!