A vest made of 15 layers of Kevlar with no steel plates could stop all but which of the following rounds?

The passage states 'It contained 15 layers of Kevlar, which could stop handgun and shotgun bullets. The vest also had a steel plate over the heart, which made the vest strong enough to stop rifle rounds.' This means the 15 layers alone stopped handgun and shotgun bullets, but rifle rounds required the additional steel plate. Therefore, without the steel plates, rifle rounds could not be stopped.