\b[U+D.] The <natural numbers> are absolute between <transitive models> of <ZFC>, and the statement $x\subseteq\mathbb N$ is the bounded assertion $\forall y\in x\,(y\in\mathbb N)$. Hence the property of being a <subset> of $\mathbb N$ is absolute.

Solution

Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 121 / 1 / i / d / Solution

 Solution (source code)