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Cognitive radio has the potential to improve spectrum efficiency and to alleviate spectrum scarcity by opportunistically utilizing un-utilized or under- utilized spectrum. A cognitive radio device needs to monitor primary user (PU) activities to identify white spaces and utilize spectral opportunities for transmission, without significantly affecting the PU performance. Additional challenges exist when PUs are reactive. An example of a reactive system is a CSMA-based primary system where PUs react to secondary user (SU) activities. Besides collision and throughput, we also introduce a deterrence metric to capture the impact of SU activity on PU. We present and compare four different SU access schemes for a CSMA-based primary system that takes into account the reactive nature of the PU access mechanism. Both simulation and analysis results show that the SU can utilize the available spectrum opportunities at the cost of additional delay of PUs.