How do you get past brooklyn-2 as am not able to clear the rubble on the street

i'm playing the PC version of fantastic 4 game and i'm stuck at the level where invisible woman has to move the rubbles...it's an alternate between 2 symbols..one is aa green sword..that is X..the other is a white shield?? where is that on the keyboard?? i pressed everything..but nothing happened..help me please!!