單項(xiàng)選擇題當(dāng)彎曲角度為90°時(shí),管子加熱長(zhǎng)度計(jì)算出來(lái)后,進(jìn)行劃線,在管端先量出至起彎點(diǎn)長(zhǎng)度a,再?gòu)腶點(diǎn)向后量出彎曲長(zhǎng)度,即量出()

A.2R
B.3R
C.1.57R
D.3.14R