[单选题]
A radar receiver indicats8xke(th 3i*f es that a transmitted pul+ 5eo8bpmv:p;cw ec u0se return as an echo in 20 ms after transmission.+ep8b0meoc;c v:upw5 How far away is the reflecting object?
A. $1.5 \; km$
B. $3.0 \; km$
C. $6.0 \; km$
D. $9.0 \; km$
