let $p:E\rightarrow B$ be a fibration with fiber $F$, ($E$ and $B$ are cw complexes). $B^k$ denotes the $k$-skeleton of $B$.
1) what does this sentence mean :"we denote the restriction of $E$ to $B^k$ by $E^k$".
2) why there is an exact sequence $\cdots \rightarrow H_{i+1}(E^{k+1},E^k)\rightarrow H_i(E^k,F)\rightarrow H_i(E^{k+1},F)\rightarrow \cdots$
3)What is a trivialization of $E$ over the $(k+1)$-cells $D_\alpha^{k+1}$ of $B^{k+1}$
from which we conclude that $H_{i+1}(E^{k+1},E^k)=H_i(\sqcup_\alpha D_\alpha^{k+1}\times F \, , \, \sqcup_\alpha S_\alpha^{k}\times F)$