挑错题(there) was （too） much dust that we （couldn't） see (what）

挑错题(there) was （too） much dust that we （couldn't） see (what）was happening.(there) was （too） much dust that we （couldn't） see (what）was A B C Dhappening.我觉得这句话没错,可是答案是b,请问b错在哪了?