How do indifference curves for perfect substitutes differ from those for perfect complements?

• A. Indifference curves for perfect substitutes are curved, while those for perfect complements are straight lines.
• B. Indifference curves for perfect substitutes are vertical, while those for perfect complements are horizontal.
• C. Indifference curves for perfect substitutes are right-angled, while those for perfect complements are curved.
• D. Indifference curves for perfect substitutes are straight lines, while those for perfect complements are right-angled.

Answer: (D)