Consider the following program, written in JavaScript$-$like syntax:Given the following calling sequences and assuming that dynamic scoping is used, what variables are visible during execution of the last subprogram activated? Include with each visible variable the name of the unit where it is declared.a$.$ main calls sub$1$; sub$1$ calls sub$2$; sub$2$ calls sub$3.$b$.$ main calls sub$1$; sub$1$ calls sub$3.$c$.$ main calls sub$2$; sub$2$ calls sub$3$; sub$3$ calls sub$1.$d$.$ main calls sub$3$; sub$3$ calls sub$1.$e$.$ main calls sub$1$; sub$1$ calls sub$3$; sub$3$ calls sub$2.$f$.$ main calls sub$3$; sub$3$ calls sub$2$; sub$2$ calls sub$1.$