MCQ
In the expansion of $(1+x)^n \cdot(1+y)^n \cdot(1+z)^n$ the sum of the coefficients of the terms of degree $r$ is:
- A$\left({ }^n \mathrm{C}_r\right)^3$
- ✓${ }^{3 n} \mathrm{C}_{\mathrm{r}}$
- C${ }^{3 \times n} C_r$
- D${ }^n \mathrm{C}_{3 \mathrm{r}}$
