A light spring balance hangs from the hook of the other light spring balance and a block of mass M kg hangs from the former one. Then the true statement about the scale reading is

Options

A. (a) Both the scales read M kg each
B. (b) The scale of the lower one reads M kg and of the upper on zero
C. (c) The reading of the two scales can be anything but the sum of the reading will be M kg
D. (d) Both the scales read `M//2kg` each

Explanation

To solve the problem, we need to analyze the system of two spring balances and the mass hanging from the lower balance.

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have two light spring balances. The first one (let's call it S1) is hanging from a hook, and the second one (S2) is hanging from S1. A block of mass \( M \) is hanging from S2.

2. **Forces Acting on the Mass**:
- The only force acting on the mass \( M \) is its weight, which is given by \( W = M \cdot g \), where \( g \) is the acceleration due to gravity.

3. **Reading of the Lower Spring Balance (S2)**:
- The reading on the lower spring balance (S2) will be equal to the weight of the mass hanging from it. Therefore, the reading \( S2 = M \cdot g \).

4. **Reading of the Upper Spring Balance (S1)**:
- The upper spring balance (S1) supports both the weight of the mass \( M \) and the reading from the lower spring balance (S2). Thus, the reading on S1 will also be equal to the total weight it supports, which is \( S1 = S2 = M \cdot g \).

5. **Conclusion**:
- Since both spring balances are massless, the readings on both balances will be equal. Therefore, we can conclude that \( S1 = S2 = M \cdot g \).

### Final Statement:
Both spring balances read the same value, which corresponds to the weight of the mass hanging from the lower balance. Hence, the correct statement is that both scales read \( M \cdot g \).