Step 1: Assume total volume of gaseous mixture.
Let the total volume of the mixture be
\[
100
\]
So, the volumes of \(A\), \(B\), and \(C\) are
\[
A=50,\quad B=30,\quad C=20
\]
Step 2: Use the first reaction.
The first reaction is
\[
A+2B \longrightarrow P_1
\]
According to the reaction,
\[
1 \text{ volume of } A
\]
reacts with
\[
2 \text{ volumes of } B
\]
Since \(B=30\), the amount of \(A\) required is
\[
\frac{30}{2}=15
\]
So, \(B\) is completely used and \(15\) volumes of \(A\) are used.
Thus, \(P_1\) formed is
\[
P_1=15
\]
Remaining \(A\) is
\[
50-15=35
\]
Remaining \(C\) is still
\[
20
\]
Step 3: Use the second reaction.
The second reaction is
\[
4P_1+3C \longrightarrow P_2
\]
For \(4\) volumes of \(P_1\), required \(C\) is \(3\) volumes.
Therefore, for \(15\) volumes of \(P_1\), required \(C\) is
\[
\frac{3}{4}\times 15
\]
\[
=\frac{45}{4}
\]
\[
=11.25
\]
Step 4: Compare available \(C\) with required \(C\).
Available \(C\) is
\[
20
\]
Required \(C\) is
\[
11.25
\]
Since
\[
20\gt 11.25,
\]
\(C\) is not completely exhausted.
But all \(15\) volumes of \(P_1\) are consumed in the second reaction.
Therefore,
\[
P_1
\]
will be completely exhausted.
Step 5: Final conclusion.
Hence, the correct statement is
\[
\boxed{P_1\text{ will be completely exhausted.}}
\]