The ubiquitous sensing-capable mobile devices have been fuelling the new paradigm of Mobile Crowd Sensing (MCS) to collect data about their surrounding environment. To ensure the timeliness and quality of the data samples in MCS, it is critical to select qualified participants to maintain sensing coverage ratios over important spatial areas (i.e., hotspots) during time periods of interest and meet various Quality of Service (QoS) requirements of sensing applications. In this paper, we examine the problems of sensing task assignment to minimize the overall cost and maximize the total utility in MCS while adhering to the QoS constraints and prove that they are NP-hard problems. Consequently, we present heuristic greedy approaches as the baseline solutions and further propose new hybrid approaches with the greedy algorithm and bees algorithm combined to address them. We demonstrate that the hybrid approaches significantly outperform the greedy approaches through extensive simulation and the analysis is given in the end.