a strike
... that a number of bus drivers who participated in a strike were unaware that it was illegally held?
... that a number of bus drivers who participated in <b id="mwAkI"><a href="./2012_Singapore_bus_drivers'_strike" id="mwAkM" rel="mw:WikiLink" title="2012 Singapore bus drivers' strike">a strike</a></b> were unaware that it was illegally held?