單項(xiàng)選擇題放張預(yù)應(yīng)力筋時(shí),混凝土強(qiáng)度必須符合設(shè)計(jì)要求,當(dāng)設(shè)計(jì)無具體要求時(shí),不應(yīng)低于設(shè)計(jì)的混凝土立方體抗壓強(qiáng)度標(biāo)準(zhǔn)值的()

A.50%
B.60%
C.70%
D.75%