The Doppler equation is $\Delta f = \frac{2 f_0 v \cos \theta}{c}$, where $\theta$ is the insonation angle between the ultrasound beam and the direction of blood flow. When the transducer is oriented parallel to blood flow, $\theta = 0^\circ$ or $180^\circ$, and $|\cos \theta| = 1$ (maximum possible value), yielding the greatest Doppler frequency shift and highest velocity measurement accuracy without angle correction errors. Thus, both (A) and (R) are correct, and (R) explains (A).

Components of radiographic developing solutions:
Hydroquinone -> Developing/reducing agent that converts exposed silver halide crystals into black metallic silver (III)

Tungsten ($W$) is the target material of choice in diagnostic X-ray tubes because of:
High atomic number ($Z = 74$) for efficient bremsstrahlung X-ray production (B)
High melting point ($3,422^\circ\text{C}$) to withstand intense electron bombardment without melting (C)
Good thermal conductivity to rapidly dissipate enormous heat loads (D)
Low vapor pressure at elevated operating temperatures (so as not to gasify and destroy the tube vacuum). Statement (A) says high vapor pressure, which is false. Thus, (B), (C), and (D) only is the correct answer.