Explain in one or two sentences why

a$)$ Large beams fail in shear at an average concrete stress that is lower than that in small beams.

b$)$ ACI allows you to design for shear sections close to a support for the same shear demand as the section $d$ from the support.

c$)$ Shear reinforcement is required where shear demand exceeds $0\mathrm{.}5V_c$ rather than $1\mathrm{.}0V_c.$

d$)$ Shear reinforcement cannot be spaced greater than $\frac{d}{2}.$