Do we add "s" to "pence"?
1)It costs 18 pence or pences?

2)And help me to do this task as well.
We're good friends. We.... each other for a long time.
a)know
b)have known
c)have been knowing
d)knew
c)had known
P.S. To my mind in this case we can use answers "b" and "c" therefore they're interchangeable, aren't they? BUt the correct answer is written to be "b"? I'm a little bit confused and don't understand why not "c"?

Question

Do we add "s" to "pence"?
1)It costs 18 pence or pences?

2)And help me to do this task as well. 
We're good friends. We.... each other for a long time.
a)know
b)have known
c)have been knowing
d)knew
c)had known
P.S. To my mind in this case we can use answers "b" and "c" therefore they're interchangeable, aren't they? BUt the correct answer is written to be "b"? I'm a little bit confused and don't understand why not "c"?

Answer

No, we don't, because it's already plural. The singular is 'penny' and the plural is 'pence'. In the past people used to say 'one penny', but now we say 'one pence'. The use of the word has changed over time to become an invariable noun, where the singular and plural are the same. We never add an 's'.

Only (b) is correct. In fact, the form (c) doesn't exist. To 'know' is a stative verb, which means it describes a state or situation, rather than an action. Stative verbs cannot be used in continuous tenses.