选择B
You must do far more than simply look.
你袖手旁观是远远不够的.
你要做的决不止是袖手旁观.
只有B far more 与must配合,最能强调鼓励人的口气.
A.some more 口气弱,与must不配.但可以说:
You should do some more than simply look.
C.much less D.any less 有否定/疑问的意思,与这句话的语境不符.