You Observed Three Different Star Clusters and Found That the Main-Sequence

You observed three different star clusters and found that the main-sequence turnoff stars in Cluster 1 had spectral type A,the main-sequence turnoff stars in Cluster 2 had spectral type B,and the main-sequence turnoff stars in Cluster 3 had spectral type G.Which star cluster is the youngest?

A) Cluster 1
B) Cluster 2
C) Cluster 3
D) It is impossible to determine their ages given only the spectral types.