If an intrinsic semiconductor is heated, the ratio of free electrons to holes is 
 

• Only the band gap will shrink as an intrinsic semiconductor is heated; the quantity of electrons and holes will remain constant.
• In an inherent semiconductor, a hole will be made for each free electron produced.
• An intrinsic semiconductor is one in which the energy gap is crossed entirely by thermal stimulation to produce electrons and holes. 
• The concentration of holes and electrons in an intrinsic semiconductor must always be the same because when electrons are excited thermally and move into the conduction band, they leave an equivalent number of vacancies or holes in the valence band.

Hence the correct answer is equal to one.