Assertion : Steel and brass are more elastic than copper and aluminium
Reason: That's why they are preferred in heavy-duty machines and in structural designs.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states: "Steel and brass are more elastic than copper and aluminum."

- **Elasticity** is measured by the modulus of elasticity (Young's modulus).
- We know from material science that steel has the highest modulus of elasticity among these materials, followed by brass, copper, and then aluminum.
- This means that steel is indeed more elastic than both copper and aluminum, but brass is not more elastic than copper.

Thus, the assertion that "Steel and brass are more elastic than copper and aluminum" is **false**.

### Step 2: Analyze the Reason
The reason states: "That's why they are preferred in heavy-duty machines and in structural designs."

- Steel is indeed preferred in heavy-duty applications due to its high modulus of elasticity, which allows it to withstand greater stresses without deforming.
- However, the statement implies that both steel and brass are preferred, which is misleading because brass is not typically used in heavy-duty applications to the same extent as steel.

Thus, the reason is also **false**.

### Step 3: Conclusion
Since both the assertion and the reason are false, we conclude that the correct answer is that both the assertion and the reason are false.

### Final Answer
Both the assertion and the reason are false.

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