Which of the following is true about ERP and legacy systems?

a$.$

ERP system can work with legacy systems for data access during transition period

b$.$

ERP system has been in use since the $1970$s$,$ but legacy system has been replacing it over the past $5$ years

c$.$

Legacy system is made by the company called Legacy Systems Inc. in California

d$.$

OLAP and OLTP exist within the legacy system, but not ERP system