Which one of the following statements is true? a. desert landscapes are monotonous, relatively flat areas covered to various depths with sand. b. deserts and dry lands are concentrated in areas of ascending air masses and relatively low atmospheric pressures. c. despite infrequent rainfalls, erosional and depositional features of running water are important in desert landscapes. d. rainshadow deserts occur where air masses descend after first having risen to cross a mountain range.