The correct option is D 1 and 2 are effects of independent causes
Choice D is correct as both are the reasons for calling on a strike and for calling off a strike, so the cause are independent. Choice A is wrong as 1 is not cause and 2 is not the effect. Choice B is wrong one is noe the cause and the other is not an effect of the first. Choice C is wrong as they clearly state that one is calling off a strike and other is calling for a strike, so they cannot be a common cause.