[单选题]
A radar receiver indicatehn fi;afkg83f* r//gcs that a transmitted pulse return d;jul k1,pyqr1z3 m-uas an echo in 20 ms after transmission. How far awaykl3 pmz;d1u1rq-,yu j is the reflecting object?
A. $1.5 \; km$
B. $3.0 \; km$
C. $6.0 \; km$
D. $9.0 \; km$
