Concept:
To determine the annual interest rate \(p\), we need enough information to form an equation containing only one unknown.
The principal amount is
\[
P = 10000
\]
and the interest is compounded quarterly.
Step 1: Analyze Statement (I) alone.
Statement (I) tells us that
\[
CI - SI = 18
\]
Although this gives a relationship between compound interest and simple interest, the time period is unknown.
Since the difference between CI and SI depends on both the rate \(p\) and the duration of investment, multiple values of \(p\) are possible.
Therefore Statement (I) alone is insufficient.
Step 2: Analyze Statement (II) alone.
Statement (II) tells us that the deposit remains invested for six months.
This means
\[
t=\frac12 \text{ year}
\]
or
\[
2 \text{ quarters}
\]
However, no information about the amount of interest earned is given.
Therefore Statement (II) alone is insufficient.
Step 3: Combine Statements (I) and (II).
From Statement (II), the investment lasts for two quarters.
Thus
\[
A=P\left(1+\frac{p}{400}\right)^2
\]
and
\[
SI=P\left(\frac{p}{100}\times \frac12\right)
=10000\left(\frac{p}{200}\right)
\]
Using Statement (I),
\[
CI-SI=18
\]
Substituting the known values produces an equation containing only \(p\).
Hence \(p\) can be uniquely determined.
Therefore both statements together are sufficient.
\[
\boxed{\text{Both statements (I) and (II) are sufficient}}
\]