You don't need to know data structures and algorithms for B and C. I'd suggest doing virtual contests if you don't like the traditional way of practicing

Simple ones like binary search and prefix sums might be useful sometimes.

I think you should practice more until you can solve B easily and fast (under 30 mins). Then you can start doing virtual contest for under-pressure-practicing.

If you find a problem is good and can be applied many times. You can do it 3 times

• First time is for understanding problem + editorial

• Second time is for doing it + high-ranker's code

• Third time is for doing faster + search for more problems similiar to it

I would suggest to practice the ladders of a2oj according to your rating range

Also check out E869120's tutorial to practice competitive programming, i found that very helpful!