Which of the following is true about incidence?

INCIDENCE Incidence rate is defined as "the number of NEW cases occurring in a defined population during a specified period of time". It will be seen from the above definition that incidence rate refers a. only to new cases b. during a given period (usually one year) c. in a specified population or "population at risk", unless other denominators are chosen. d. it can also refer to new spells or episodes of disease arising in a given period of time, per 1000 population. Ref : Park 23rd edition Pgno : 59