Any tips on focusing on something that you don't like and only half understand.

Choose a thought. This can be anything.  Hold on to this thought for 5 minutes.  You should not let any other thought to creep in.  You can make this a daily habit so you can switch to high concentration at will. Increase the length to about 10 minutes and focus on a single thought for this amount of time. Set small goals, Use sticky notes and make to-do lists,  Meditate, Work-out, Memorize,  Read, Keep learning.

option c (nickel) is not found in earth's crust. the earth's crust basically contains eight (8) elements namely: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium (listed in order of abundance).