The main memory unit with a capacity of $4$ $\text{megabytes}$ is built using $1\text{M} \times \text{1-bit}$ DRAM chips. Each DRAM chip has $1\text{K}$ rows of cells with $1\text{K}$ cells in each row. The time taken for a single refresh operation is $100\; \text{nanoseconds}$. The time required to perform one refresh operation on all the cells in the memory unit is
- $100$ nanoseconds
- $100\times 2^{10}$ nanoseconds
- $100\times 2^{20}$ nanoseconds
- $3200\times 2^{20}$ nanoseconds