EMPLOYMENT HISTORY RFE ON STEM EXTENSION OPT

Hi,I got an rfe stating 1.Submit a complete employmn,ent history from June 4,2012 through April 30,2013.Include name of each employer,dates of employment,title of position,description of job duties,hours worked per week and salary.Include pay stubs and pay summaries.Submit all your form W2(s) for year 2012 So how should i respond 1.Should i include offer letters? I have worked as unpaid volunteer for 4 months at the university.I joined compay A in July 23,2013 to Sept 2,2013 company B in jan 30 to April 22,2013.Currently i am working with everified full time company C from April 30,2013 to present .I applied under STEM Extension under company C Please help me with this its urgent