Which of the following is a difference between primary data and secondary data?

a$.$ Primary data are outdated, whereas secondary data are current and have known sources.

b$.$ Primary data are inexpensive to gather, while secondary data are expensive to gather.

c$.$ Primary data can be maintained in secrecy$,$ while secondary data are usually available to all interested parties for relatively small fees or free.

d$.$ Primary data are data that have been previously collected, while secondary data are data that have been collected for the first time.