multicharacter character constant warning

    if(per>=80)
  grade='A+';
    else if(per>=75&&per<=79)
  grade='A';
    else if(per>=70&&per<=74)
  grade='A-';
    else if(per>=65&&per<=69)
  grade='B+';
    else if(per>=60&&per<=64)
  grade='B';
    else if(per>=55&&per<=59)
  grade='B-';
    else if(per>=50&&per<=54)
  grade='C+';
    else if(per>=45&&per<=49)
  grade='C-';
    else if(per>=40&&per<=44)
  grade='D';
    else
            grade='F';

when i run this code i receive the message multicharacter character constant.I see previous solutions of the same problem but failed to implement it in my code.Please any one help me and tell me what should i use to run the code