ارجو من الاساتذة الكبار توضيح الفرق بين can and could فى حالة possibility

[djdjdj]

I am confused When I can use( can) and (could)in the possibility because I know that both of them used in the future and the present or can I use (could)in the past? I want to know the reason for the following sentences

Driving a car while we talking on a mobile phone ....be dangerous
a-could b-can not c-can d-could not.

Without my glasses I can't see what that is on the wall,but it .....be a spider
a-could b-can not c-can d-could not

Thanxxxxxxxxxx