The problem describes a pollution scenario where initially there are 7 pieces of rubbish on a beach. Each week, the number of pieces of rubbish triples. We need to find the expression that represents the number of pieces of rubbish after 5 years (which is equivalent to 5 weeks since the tripling happens each week).

1. Problem Description

The problem describes a pollution scenario where initially there are 7 pieces of rubbish on a beach. Each week, the number of pieces of rubbish triples. We need to find the expression that represents the number of pieces of rubbish after 5 years (which is equivalent to 5 weeks since the tripling happens each week).

2. Solution Steps

The initial number of rubbish pieces is

7. The number of rubbish pieces triples each week. Tripling means multiplying by

3. After 1 week, the number of rubbish pieces is $7 \times 3$.

After 2 weeks, the number of rubbish pieces is $7 \times 3 \times 3 = 7 \times 3^2$.

After 3 weeks, the number of rubbish pieces is $7 \times 3 \times 3 \times 3 = 7 \times 3^3$.

Following this pattern, after 5 weeks, the number of rubbish pieces is $7 \times 3^5$.

This corresponds to option C.

3. Final Answer

C. $7(3)^5$