It took him a long time to ______ the skills needed tobecome a professional artist

I wondered what difficulty he had ______ the plan?[ ]

What great changes! It’s years since we_____ each other last time — Believe it or not, It’s the second time that we _____a face –to- face talk

在市场上购买衣服时，看到的表示“中”号的字母。

The subway is too because there are so many people in it.