**Answer: A) 9**

**Explanation:**

First of all, we need to compute the traffic intensity in Erl.

If we take the busy hour, for 60 calls of 60 sec each, we would have 100% occupied the circuit, which we define as 1 Erl.

Making a simple comparison, if we have half of calls, with half duration each, we will have the circuit busy 25% of time, i.e. 0.25 Erl.

The CCS is a unit of telephony traffic, that measures it in units of 100 call-seconds (calls that last 100 seconds), and is equivalent to 1/36 Erl, due to we can show that 1 Erl = 36 CCS

(If we had 36 calls of 100 sec each (100 CCS), we would have the circuit busy 100% time, i.e., with 1 Erl traffic).

If we know then that 1 Erl= 36 CCS, we can find out how many CCSs are equivalent to 0.25 Erl, solving a simple direct proportion, as follows:

36 CCS/ 1 Erl = x CCS/ 0.25 Erl ⇒ **9 CCS**