In dense fog a vessel without operational radar may not be justified ________ at all but should anchor if it is safe and practicable for her to do so．

(航海日志)1105 拖船Satex系右首，按船长和引航员命令用车和拖船。

(航海日志)0400 晴，海面中浪，巡视全船正常。推算船位50-00’.2N，001-40’.0W，计程仪读数67’.2。

我们试图放下救生艇，但没有成功。

如果明天下雨，我们将停止卸货。

船舶不得在分道通航制区域内抛锚。

This paragraph is ______ ．

DISCRIMINATION means the ______ ．

The eye is more ______ than radar．

The block is not used to ______ ．

材料：

What advantages does the eye have over the radar ？

The eye has the following advantages:

Reliable

Sensitive to color

Can assess heading

Can identify small targets

Can see light configurations

Can assess ship types

Can identify conspicuous marks

Can identify flashing lightsHas better discrimination

Can see changing weather patterns

Can see effect of sea on vessel

Not affected by blind arcs (if observer moves)．

问题：BLIND ARCS mean ______ ．