[单选题]
A radar receiver indicatesk2s71asp:bci una 0sqfz0m h8 7oqu4 4 that a tranesr 3r-aq7fq ( nl45nfsmitted pulse return as an echo in 20 ms after transmission. How far away is th4(s7 f5rla3-fqnr neq e reflecting object?
A. $1.5 \; km$
B. $3.0 \; km$
C. $6.0 \; km$
D. $9.0 \; km$
