[单选题]
A radar receiver indicates that a transmitted pulse return al s8hf; 5/:easbzri(oo .jix+s an1ite* / d7rdh 79adykc echo in 20 ms after transmission. How far away is the reflert*/d i9dh 7 ycd1ek7acting object?
A. $1.5 \; km$
B. $3.0 \; km$
C. $6.0 \; km$
D. $9.0 \; km$
