按GB7588规定，层门锁紧装置应能防止妨碍正常功能的积尘，工作部件应便于检查，例如采用一块透明板按GB7588规定，层门锁紧装置应能防止妨碍正常功能的积尘，工作部件应便于检查，例如采用一块透明板以便观察。

l not attack ________ we are attacked; if attacked, we will certainly counter-attack. [A] if [B] when [C] unless [D] even if