请选择DCBA8L+\,r2s,W,H3^6L1j/} One of the boys _______ reading a book under the tree.(l6{3N8T+\6~8S-I1?:f1k
A. am(l6{3N8T+\6~8S-I1?:f1k
B. is(l6{3N8T+\6~8S-I1?:f1k
C. be(l6{3N8T+\6~8S-I1?:f1k
D. are(l6{3N8T+\6~8S-I1?:f1k