If $a, b$ and $n$ are natural numbers, then $a^{2n-1} + b^{2n-1}$ is always divisible by:

The remainder on dividing $5^{99}$ by $11$ is

The remainder when $(2021)^{2023}$ is divided by $7$ is

For any natural number $n$,$(15 \times 5^{2n}) + (2 \times 2^{3n})$ is divisible by

The remainder,when $3^{2003}$ is divided by $28$,is

For all positive integers $n$,if $3(5^{2n+1}) + 2^{3n+1}$ is divisible by $k$,then the number of prime numbers less than or equal to $k$ is: