Which statement about ETL for data warehouse projects is$$not correct?

$($a$)$ It usually takes about $60-80\%$ of all the effort

$($b$)$ Most ETL activities are performed at the staging$$database area

$($c$)$ ETL stand for Extract Transform Load

$($d$)$ Surrogate keys of dimensions are generated during$$the RTL stage and then surrogate keys are inserted$$into the fact table first and then into the dimension$$tables$.]$