---I’m really _________ before the exam.

---Take it easy. You’re the best.

A．surprised

B．nervous

C．comfortable

D．confident

数列{a_n}、{b_n}都是等差数列，a₁=5，b₁=7，且a₂₀+b₂₀=60．则{a_n+b_n}的前20项和为（　　）

A．700

B．710

C．720

D．730