求英语达人告诉我答案以及为什么选这个答案.要详细一点.Before the first non-stop flight was made in 1949,it_____necessary for all planes to land for refueling.A.would be B.has been.C.had been D.would have been

来源:学生作业帮助网 编辑:作业帮 时间:2024/09/01 15:29:39
xSNQ~%MzcBLGeޡ .)Hh+j]9sp+tvϊJ4mtf}732oR:|2 {E+0mٱF6E:0Cm):^mޙm co%O*$QH8LW Ǣ4 ț@H!Q<_z$EӯOU^+T `H"PI !97XT]_ʛX"GfPQTy͐k5/ x/`wa%׌}FMh]]Zv3kHNH >QKPHCG`_d *7u5=iLzN>k0uk 12[g0҉;wK jJٮe0sF&7 ֭mu@3Dyda܇u>#~֨:[%&5"]VAƒ Ht|'ń/- Ɖ{aMUڕYMV:gFU]Ej[;ܸm@DWhΐWl@;fvRݡY%E~