question: What are the three operation modes proposed for LTE-U networking? option 1: Supplementary downlink (SDL), time division LTE (TD-LTE), standalone LTE-U option 2: Supplementary downlink (SDL), carrier aggregation (CA), carrier sensing adaptive transmission (CSAT) option 3: Supplementary uplink (SUL), carrier aggregation (CA), standalone LTE-U option 4: Supplementary downlink (SDL), carrier aggregation (CA), standalone LTE-U option 5: Supplementary downlink (SDL), carrier aggregation (CA), dynamic frequency selection 