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

Good day,
the difference is presented in longmansec site in detail. Please have a look here:
http://www.longmansec.com.eg/QuestAns.aspx?QA_ID=1528
Hope it helps

I still can not differentiate between them .Can you elaborate more?
Thanxxxx

سأضع لك شرح مفصل لكل استخداماتها إن شاء الله الليلة
لا تقلق فقط تابعنى إن شاء الله

I hope any teacher can answer me

ستجد الموضوع هنا على الوصلة التالية :
http://thanwya.com/vb/showthread.php?p=2161728#post2161728

There are two types of possibility. We can use can or could to say that "things are possible in general: that people are able to do them, the situation makes them possible, or there is nothing to stop them." (Swan p320)

These plants can grow in any type of soil.
In the past, everybody could ride a horse.

We can also talk about a degree of certainty. "To talk about the chance (possibility) that something will happen, or is happening, we use may, might or could, but not can." (Swan p 320)
Where's Mona? ~ She may be in class. (NOT She can be …)
We may go to Luxor next summer. (NOT We can go …)
"Might and could suggest a less strong possibility." (Swan p320)
It might/could snow today, but I think it probably won't. (NOT It can rain …)

Can is rarely used in the positive to express a degree of certainty, and only then if it is qualified. Do not use can in this way on an exam. We use could (and other modals) to express a degree of certainty. It is possible to use can in the negative to say that it is impossible for something to be true.
That can't be John at the door. He's travelling today.
(Michael Swan, Practical English Usage, 3rd ed.)