Which of the balance of payments accounts $($current account or capital financial account$)$ do the following events affect? For the US balance of payments, would this transaction be considered a credit $(+)$ or a debit $(-)?$

a$.$ Boeing, a U$.$S$.-$based company, sells a newly built airplane to China.

b$.$ Chinese investors buy stock in Boeing from Americans.

c$.$ A Chinese company buys a used airplane from American Airlines and ships it to China.

d$.$ A Chinese investor who owns property in the United States buys a corporate jet, which he will keep in the United States so he can travel around America.