Consider the following program, writ ten in JavaScript-like syntax: // main prograrm var x, Y. 2 function subl) var a,yz function sub2 ) var a, b, z; function sub3 ) vara, x, w; Given the following calling sequences and assuming that dynamic scop- ing is used, what variables are visible during execution of the last subpro- gram activated? Include with each visible variable the name of the unit where it is declared. a. main calls subl; subl calls sub2; sub2 calls sub3 b. main calls subl; subl calls sub3 c. main calls sub2; sub2 calls sub3; sub3 calls sub1. d. main calls sub3; sub3 calls subl. e. main calls subl; subl calls sub3; sub3 calls sub2 f. main calls sub3; sub3 calls sub2; sub2 calls sub1