Assume a network that consists of a set of nodes, $\{$a$,$ b$,$ c$,$ d$,$ e$,$ f$,$ g$,$ h$,$ j$,$ k$,$ l$,$ x$,$ z$\}.$ These nodes have a partial ordering relation, $<=,$ defined as follows:

$\{($b$<=$a$),($f$<=$e$),($z$<=$l$),($z$<=$x$),($l$<=$g$),($c$<=$b$),($g$<=$k$),($e$<=$d$),($g$<=$h$),($k$<=$j$),$

$($g$<=$e$),($e$<=$c$),($d$<=$b$),($x$<=$e$)\}$

And therefore, the ordering relation would state that b is less important than a$,$ f less important than e and so on$.$ Furthermore, assume that a BLP policy is being enforced in the above network. Now, assume that at some stage, the node z becomes infected with some kind of a virus program.

Which one of the following sets of actions would then lead to infecting node a$,$ assuming that viruses propagate through a network using read and write commands. A virus would propagate from one node to another either because the second node read the virus code from a file stored on the first one, or because the first node wrote the virus code onto a file present in the second one. All read and write commands are subject to the policy being enforced $($i$.$e$.$ BLP in this case$)$ and no read or write operation is possible in the absence of an order, either directly or indirectly, between two nodes. Hence for example, h cannot read or write to f as there is no direct or indirect ordering between the two $($and so they are incomparable$)$:

a$)($l read from z$),($l write to g$),($g read from l$),($g read from e$),($e write to c$),($c write to b$),($a read from b$)$

b$)($z read from l$),($a read from l$)$

c$)($z write to x$),($x write to e$),($a read from e$)$

d$)($z write to c$),($b write to c$),($a read from b$)$

e$)($z write to g$),($g write to h$),($b read from h$),($b write to a$)$

f$)($x read from z$),($x write to e$),($e write to d$),($a write to d$)$

g$)($z write to g$),($j read from g$),($j write to d$),($d write to b$),($a read from b$)$

h$)($z write to l$),($l write to g$),($g write to c$),($c write to d$),($d write to b$),($b write to a$)$ i$)($a write to z$)$

j$)($f read from z$),($a read from f$)$