Choose the correct sentence (advanced grammar):

One who can speak two languages:

I wanted to go, _______ I was too tired.

The book _____ by millions of readers worldwide.

Read the passage: \'Renewable energy sources like solar and wind now account for 30% of global electricity. However, their intermittency requires advanced battery storage systems to ensure consistent supply.\' What is the main challenge mentioned?

The children were playing in the park _____ it started to rain.