Before revision:  In contrast to previous work, advisor-student and student-co-advisor pairs may not be mutually acceptable , e.g., a student may not want to work with an advisor or co-advisor and vice versa  .
After revision:  In contrast to previous work, advisor-student and student-co-advisor pairs may not be mutually acceptable , e.g., a student may not want to work with an advisor or co-advisor and vice versa ) .