20. A network consists of 16 vertices and 24 edges that connect them, as shown. An ant begins at the vertex labelled Start. Every minute, it walks from one vertex to a neighbouring vertex, crawling along a connecting edge. At which of the vertices labelled $P, Q, R, S, T$ can the ant be after 2019 minutes?
A only $P, R$ or $S$,
$\mathrm{B}$ not $Q$
C only $Q$
D only $T$
$E$ all of the vertices are possible
20. C Labelling vertices alternately $0 / 1$ leads to the labelling shown. After an odd number of steps, the ant is always on a vertex labelled 1. The only such vertex labelled with a letter is $Q$.