I don't get why the pricing is based on contacts. Why not charge based on emails sent? This is probably where most of your expenses are, instead of charging on storing contacts in the DB and querying them.
I have a SaaS B2C with more than 10k contacts and I don't send lots of emails.
1. In The Long Tail > Nested Queries, the first example contains the function LEN, which gives an error as LENGTH is expected.
2. On the questions with multiple answers, when showing hints, I would use different font colors for correct/incorrect statements as it will be easier to check your results.
