If the following graph represents the decay of strontium-90 in an igneous rock, what event occured at 0 years on the horizontal axis?

“the battle of stalingrad is not a turning point necessarily in strategic terms, because a lot more has to be done before the soviets can be certain of defeating germany,” says renowned world war ii historian richard overy. the germans had fought nearly a thousand miles across the soviet union to get to stalingrad.

what are the options ?