Which of the following is true for smart contracts?

Group of answer choices

$($a$)$ They are software code outlining contracts associated with transactions, which remain unaltered, accessible and publicly verifiable for the blockchain$$s lifespan.

$($b$)$ Smart contracts often interface with external applications that reside outside the blockchain

$($c$)$ Smart contracts can be used for peer$-$to$-$peer transactions, e$.$g$.,$ smart electricity grid, ridesharing services, etc

$($d$)$ All of the above