Unlike placental mammals, both monotremes and marsupials:

Options

A. lack nipples
B. have some embryonic development outside the mother's uterus
C. lay eggs
D. are found in Australia and Africa

Explanation

To answer the question "Unlike placental mammals, both monotremes and marsupials:", we need to analyze the characteristics of monotremes and marsupials in comparison to placental mammals.

### Step-by-Step Solution:

1. **Understanding the Groups**:
- **Monotremes**: These are egg-laying mammals, such as the platypus and echidnas. They belong to the subclass Prototheria.
- **Marsupials**: These mammals, like kangaroos and possums, give birth to underdeveloped young that continue to develop in a pouch (marsupium). They belong to the infraclass Metatheria.
- **Placental Mammals**: These mammals have a true placenta and their young develop entirely within the uterus until a more advanced stage.

2. **Analyzing the Options**:
- **Option A: Lack Nipples**: This is incorrect. Both monotremes and marsupials have mammary glands and can produce milk, although monotremes do not have nipples in the traditional sense.
- **Option B: Have Some Embryonic Development Outside the Mother's Uterus**: This is correct. Monotremes lay eggs, and marsupials give birth to underdeveloped young that continue to develop in the pouch.
- **Option C: Lay Eggs**: This is true only for monotremes, not for marsupials, as marsupials give live birth.
- **Option D: Found in Australia and Africa**: While monotremes are primarily found in Australia, marsupials are also found in other regions, so this statement is not universally true.

3. **Conclusion**:
- The correct answer is **Option B**: "Have some embryonic development outside the mother's uterus," as this characteristic applies to both monotremes and marsupials, distinguishing them from placental mammals.