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For entities "Customer" $($E$1)$ and "Order" $($E$3)$ in an ecommerce database, what relationship type is appropriate?

a$.$ N$-$M $($Many$-$to$-$Many$)$

b$.N-1($Many$-$to$-$One$)$

c$.1-$N $($One$-$to$-$Many$)$

d$.1-1($One$-$to$-$One$)$